<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-03-15</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001326003</issuerCik>
        <issuerName>BlackRock Kelso Capital CORP</issuerName>
        <issuerTradingSymbol>BKCC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001365234</rptOwnerCik>
            <rptOwnerName>Gordon Frank D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O BLACKROCK KELSO CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>40 EAST 52ND STREET</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle>Chief Financial Officer</officerTitle>
            <otherText>C.F.O. of investment adviser</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.001 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>11737</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>43690.8</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Restricted Shares granted by BlackRock Kelso Capital Advisors LLC (&quot;Advisor&quot;) pursuant to a Restricted Stock Agreement, dated March 15, 2010, vesting in equal installments on January 2, 2011, 2012 and 2013.</footnote>
        <footnote id="F2">Includes 1,101.52 Restricted Shares granted by Advisor pursuant to a Restricted Stock Agreement, dated December 31, 2007, vesting on January 2, 2011; 3,221 Restricted Shares granted by Advisor pursuant to a Restricted Stock Agreement, dated January 2, 2009, vesting in equal installments on January 2, 2011 and 2012; and 1,610 Restricted Shares granted by Advisor pursuant to a Restricted Stock Agreement, dated January 23, 2009, vesting in equal installments on January 2, 2011 and 2012.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Frank D. Gordon</signatureName>
        <signatureDate>2010-03-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
