Exhibit 5.1

		Goodwin Procter LLP
		100 Northern Avenue
		Boston, MA 02210
		goodwinlaw.com
		+1 617 570 1000
	January 23, 2020

Blueprint Medicines Corporation

45 Sidney Street

Cambridge, MA 02139

Re:           Securities Registered under Registration Statement on Form S-3ASR

Ladies and Gentlemen:

We have acted as counsel to Blueprint Medicines Corporation, a Delaware corporation (the “Company”) in connection with its filing of a Registration Statement on Form S-3ASR (File No. 333-216573) (as amended or supplemented, the “Registration Statement”) filed on March 9, 2017 with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”). The Registration Statement became effective under the Securities Act upon filing on March 9, 2017.  Reference is made to our opinion letter dated March 9, 2017 and included as Exhibit 5.1 to the Registration Statement. We are delivering this supplemental opinion letter in connection with the prospectus supplement (the “Prospectus Supplement”) filed on January 22, 2020 by the Company with the Commission pursuant to Rule 424 under the Securities Act. The Prospectus Supplement relates to the offering by the Company of up to 5,416,665 shares of the Company’s Common Stock, par value $0.001 per share (the “Shares”) covered by the Registration Statement. The Shares include an overallotment option granted to the underwriters of the offering to purchase up to an additional 706,521 Shares.  The Shares are being sold to the several underwriters named in, and pursuant to, an underwriting agreement among the Company and such underwriters (the “Underwriting Agreement”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinion set forth below.  We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

The opinion set forth below is limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, upon issuance and delivery against payment therefor in accordance with the terms of the Underwriting Agreement, the Shares will be validly issued, fully paid and non-assessable.

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Registration Statement and to the references to our firm under the caption “Legal Matters” in the Registration Statement.

In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

Very truly yours,

/s/ Goodwin Procter LLP

GOODWIN PROCTER LLP