BLACKROCK ALLOCATION TARGET SHARES

BATS: Series C Portfolio

BATS: Series M Portfolio

BATS: Series S Portfolio

(each, a “Fund” and collectively, the “Funds”)

Supplement dated October 29, 2021 to

the Statement of Additional Information of the Funds,

dated July 29, 2021, as supplemented to date

The Board of Trustees of BlackRock Allocation Target Shares approved certain changes to the Funds’ fundamental investment restrictions, which required shareholder approval. At a special meeting held on October 26, 2021, shareholders of each Fund approved the changes to the Fund’s fundamental investment restrictions.

Effective November 30, 2021 the subsection of the Statement of Additional Information entitled “Investment Restrictions – Fundamental Investment Restrictions (each Fund except for BATS: Series A Portfolio, BATS: Series E Portfolio and BATS: Series P Portfolio)” is deleted in its entirety and replaced with the following:

Fundamental Investment Restrictions (each Fund except for BATS: Series A Portfolio, BATS: Series E Portfolio and BATS: Series P Portfolio)

The investment restrictions set forth below are fundamental policies of each of the Funds (except for BATS: Series A Portfolio, BATS: Series E Portfolio and BATS: Series P Portfolio) and may not be changed with respect to any of the Funds without the approval of the holders of a majority of the Fund’s outstanding voting securities (which for this purpose and under the Investment Company Act means the lesser of (i) 67% of the shares represented at a meeting at which more than 50% of the outstanding shares are represented or (ii) more than 50% of the outstanding shares).

Under these fundamental investment restrictions, none of the Funds may:

1. Concentrate its investments in a particular industry, as that term is used in the Investment Company Act.

2. Borrow money, except as permitted under the Investment Company Act.

3. Issue senior securities to the extent such issuance would violate the Investment Company Act.

4. Purchase or hold real estate, except the Fund may purchase and hold securities or other instruments that are secured by, or linked to, real estate or interests therein, securities of real estate investment trusts, mortgage related securities and securities of issuers engaged in the real estate business, and the Fund may purchase and hold real estate as a result of the ownership of securities or other instruments.

5. Underwrite securities issued by others, except to the extent that the sale of portfolio securities by the Fund may be deemed to be an underwriting or as otherwise permitted by applicable law.

6. Purchase or sell commodities or commodity contracts, except as permitted by the Investment Company Act.

7. Make loans to the extent prohibited by the Investment Company Act.

Notations Regarding each Fund except for BATS: Series A Portfolio, BATS: Series E Portfolio and BATS: Series P Portfolio

The following notations are not considered to be part of a Fund’s fundamental investment restrictions and are subject to change without shareholder approval.

With respect to the fundamental policy relating to concentration set forth in (1) above, the Investment Company Act does not define what constitutes “concentration” in an industry. The Commission staff has taken the position

that investment of 25% or more of a fund’s total assets in one or more issuers conducting their principal activities in the same industry or group of industries constitutes concentration. It is possible that interpretations of concentration could change in the future. The policy in (1) above will be interpreted to refer to concentration as that term may be interpreted from time to time. The policy also will be interpreted to permit investment without limit in the following: securities of the U.S. government and its agencies or instrumentalities; securities of state, territory, possession or municipal governments and their authorities, agencies, instrumentalities or political subdivisions; and repurchase agreements collateralized by any such obligations. Accordingly, issuers of the foregoing securities will not be considered to be members of any industry. There also will be no limit on investment in issuers domiciled in a single jurisdiction or country. Finance companies will be considered to be in the industries of their parents if their activities are primarily related to financing the activities of the parents. Each foreign government will be considered to be a member of a separate industry. With respect to the Fund’s industry classifications, the Fund currently utilizes any one or more of the industry sub-classifications used by one or more widely recognized market indexes or rating group indexes, and/or as defined by Fund management. The policy also will be interpreted to give broad authority to the Fund as to how to classify issuers within or among industries.

With respect to the fundamental policy relating to borrowing money set forth in (2) above, the Investment Company Act permits the Fund to borrow money in amounts of up to one-third of the Fund’s total assets from banks for any purpose, and to borrow up to 5% of the Fund’s total assets from banks or other lenders for temporary purposes. (The Fund’s total assets include the amounts being borrowed.) In addition, the Fund has received an exemptive order from the SEC permitting it to borrow through the Interfund Lending Program (discussed below), subject to the conditions of the exemptive order. To limit the risks attendant to borrowing, the Investment Company Act requires the Fund to maintain at all times an “asset coverage” of at least 300% of the amount of its borrowings. Asset coverage means the ratio that the value of the Fund’s total assets (including amounts borrowed), minus liabilities other than borrowings, bears to the aggregate amount of all borrowings. Borrowing money to increase portfolio holdings is known as “leveraging.” Certain trading practices and investments, such as reverse repurchase agreements, may be considered to be borrowings or involve leverage and thus are subject to the Investment Company Act restrictions. In accordance with Commission staff guidance and interpretations, when the Fund engages in such transactions, the Fund instead of maintaining asset coverage of at least 300%, may segregate or earmark liquid assets, or enter into an offsetting position, in an amount at least equal to the Fund’s exposure, on a mark-to-market basis, to the transaction (as calculated pursuant to requirements of the Commission). The policy in (2) above will be interpreted to permit the Fund to engage in trading practices and investments that may be considered to be borrowing or to involve leverage to the extent permitted by the Investment Company Act and to permit the Fund to segregate or earmark liquid assets or enter into offsetting positions in accordance with the Investment Company Act. Short-term credits necessary for the settlement of securities transactions and arrangements with respect to securities lending will not be considered to be borrowings under the policy. Practices and investments that may involve leverage but are not considered to be borrowings are not subject to the policy.

With respect to the fundamental policy relating to underwriting set forth in (5) above, the Investment Company Act does not prohibit the Fund from engaging in the underwriting business or from underwriting the securities of other issuers; in fact, in the case of diversified funds, the Investment Company Act permits the Fund to have underwriting commitments of up to 25% of its assets under certain circumstances. Those circumstances currently are that the amount of the Fund’s underwriting commitments, when added to the value of the Fund’s investments in issuers where the Fund owns more than 10% of the outstanding voting securities of those issuers, cannot exceed the 25% cap. A fund engaging in transactions involving the acquisition or disposition of portfolio securities may be considered to be an underwriter under the Securities Act of 1933, as amended (the “Securities Act”). Although it is not believed that the application of the Securities Act provisions described above would cause the Fund to be engaged in the business of underwriting, the policy in (5) above will be interpreted not to prevent the Fund from engaging in transactions involving the acquisition or disposition of portfolio securities, regardless of whether the Fund may be considered to be an underwriter under the Securities Act or is otherwise engaged in the underwriting business to the extent permitted by applicable law.

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With respect to the fundamental policy relating to lending set forth in (7) above, the Investment Company Act does not prohibit the Fund from making loans (including lending its securities); however, Commission staff interpretations currently prohibit funds from lending more than one-third of their total assets (including lending its securities), except through the purchase of debt obligations or the use of repurchase agreements. In addition, collateral arrangements with respect to options, forward currency and futures transactions and other derivative instruments (as applicable), as well as delays in the settlement of securities transactions, will not be considered loans.

The Fund is currently classified as a diversified fund under the Investment Company Act. This means that the Fund may not purchase securities of an issuer (other than (i) obligations issued or guaranteed by the U.S. government, its agencies or instrumentalities and (ii) securities of other investment companies) if, with respect to 75% of its total assets, (a) more than 5% of the Fund’s total assets would be invested in securities of that issuer or (b) the Fund would hold more than 10% of the outstanding voting securities of that issuer. With respect to the remaining 25% of its total assets, the Fund can invest more than 5% of its assets in one issuer. Under the Investment Company Act, the Fund cannot change its classification from diversified to non-diversified without shareholder approval.

Non-Fundamental Investment Objective and Restrictions (each Fund except for BATS: Series A Portfolio, BATS: Series E Portfolio and BATS: Series P Portfolio)

The investment objective of each Fund is not fundamental and may be changed without shareholder approval. Each Fund is also subject to the following non-fundamental restrictions (which may be changed without shareholder approval) and, unless otherwise indicated, may not:

(a) Purchase securities of other investment companies, except to the extent permitted by the Investment Company Act. As a matter of policy, however, the Fund will not purchase shares of any registered open-end investment company or registered unit investment trust, in reliance on Section 12(d)(1)(F) or (G) (the “fund of funds” provisions) of the Investment Company Act, at any time the Fund has knowledge that its shares are purchased by another investment company investor in reliance on the provisions of subparagraph (G) of Section 12(d)(1).

Unless otherwise indicated, all limitations applicable to a Fund’s investments apply only at the time a transaction is entered into. Any subsequent change in a rating assigned by any rating service to a security, or change in the percentage of a Fund’s assets invested in certain securities or other instruments resulting from market fluctuations or other changes in a Fund’s total assets, will not require the Fund to dispose of an investment. In the event that ratings services assign different ratings to the same security, the investment adviser will determine which rating it believes best reflects the security’s quality and risk at that time, which may be the higher of the several assigned ratings.

To comply with U.S. Federal tax requirements for qualification as a “regulated investment company,” each Fund’s investments will be limited in manner such that at the close of each quarter of each tax year, subject to certain exceptions and cure periods, (a) no more than 25% of the value of a Fund’s total assets are invested in the securities (other than U.S. Government securities or securities of other regulated investment companies) of a single issuer, any two or more issuers controlled by the Fund and engaged in the same, similar or related trade or businesses or certain publicly traded partnerships and (b) with regard to at least 50% of the Fund’s total assets, no more than 5% of its total assets are invested in the securities (other than U.S. Government securities or securities of other regulated companies) of a single issuer and no investment will consist of 10% or more of the voting stock of such issuer. These tax-related limitations may be changed by the Trustees to the extent appropriate in light of changes to applicable tax requirements.

Shareholders should retain this Supplement for future reference.

SAI-BATS-1021SUP

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