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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-12-20</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001000683</issuerCik>
        <issuerName>BLONDER TONGUE LABORATORIES INC</issuerName>
        <issuerTradingSymbol>BDR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001204017</rptOwnerCik>
            <rptOwnerName>NECESSARY STEPHEN K</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O BLONDER TONGUE LABORATORIES, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>ONE JAKE BROWN ROAD</rptOwnerStreet2>
            <rptOwnerCity>OLD BRIDGE</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>08857</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.001</value>
            </securityTitle>
            <transactionDate>
                <value>2019-12-20</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>23893</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.655</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>67126</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the company's Amended and Restated Director Stock Purchase Plan, the reporting person has elected to receive a fully vested stock award for shares of the company's common stock in lieu of cash payment of director fees otherwise payable to the reporting person. The number of shares of common stock awarded represents the quotient of (i) the director fees payable to the reporting person divided by (ii) $0.655, representing the average of the high and low trading prices reported on the NYSE American on December 20, 2019.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Stephen K. Necessary</signatureName>
        <signatureDate>2019-12-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
