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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-09-11</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001757536</rptOwnerCik>
            <rptOwnerName>Grauch Edward Rowland</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>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>CEO &amp; President</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.001</value>
            </securityTitle>
            <transactionDate>
                <value>2020-09-11</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4603</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.84</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>280423</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to Deferred Compensation Agreements between the reporting person and the issuer, on the date on which a portion of the reporting person's compensation has been deferred, the issuer accrued the number of shares indicated. The number of shares of common stock accrued represents the quotient of (i) the portion of compensation deferred divided by (ii) the fair market value per share of the issuer's common stock on the indicated date (equal to the closing price reported on the NYSE American on such date).</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/Edward R. Grauch by Eric Skolnick under Power of Attorney</signatureName>
        <signatureDate>2020-09-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
