<SUBMISSION>
<ACCESSION-NUMBER>0001091171-17-000024
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20170217
<FILING-DATE>20170217
<DATE-OF-FILING-DATE-CHANGE>20170217
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>BLUE NILE INC
<CIK>0001091171
<ASSIGNED-SIC>5944
<IRS-NUMBER>911963165
<STATE-OF-INCORPORATION>DE
<FISCAL-YEAR-END>0101
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>411 FIRST AVE S
<STREET2>STE 700
<CITY>SEATTLE
<STATE>WA
<ZIP>98104
<PHONE>2063366700
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>411 FIRST AVE S
<STREET2>STE 700
<CITY>SEATTLE
<STATE>WA
<ZIP>98104
</MAIL-ADDRESS>
<FORMER-COMPANY>
<FORMER-CONFORMED-NAME>INTERNET DIAMONDS INC
<DATE-CHANGED>20000131
</FORMER-COMPANY>
</ISSUER>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>VAN SCHOONENBERG ROBERT G
<CIK>0001197590
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-50763
<FILM-NUMBER>17622317
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>150 N. ORANGE GROVE BLVD
<CITY>PASADENA
<STATE>CA
<ZIP>91103
</MAIL-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_148736960890681.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-02-17</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001091171</issuerCik>
        <issuerName>BLUE NILE INC</issuerName>
        <issuerTradingSymbol>NILE</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001197590</rptOwnerCik>
            <rptOwnerName>VAN SCHOONENBERG ROBERT G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O BLUE NILE, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>411 FIRST AVENUE S, SUITE 700</rptOwnerStreet2>
            <rptOwnerCity>SEATTLE</rptOwnerCity>
            <rptOwnerState>WA</rptOwnerState>
            <rptOwnerZipCode>98104</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</value>
            </securityTitle>
            <transactionDate>
                <value>2017-02-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>10751</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the agreement and plan of merger dated as of November 6, 2016 (&quot;Merger Agreement&quot;), by and among the issuer, Blue Nile, Inc., a Delaware corporation, BC Cyan Parent Inc., a Delaware corporation, and BC Cyan Acquisition Inc., a Delaware corporation and a wholly-owned subsidiary of BC Cyan Parent Inc., at the Effective Time (as defined in the Merger Agreement), each outstanding share of common stock and restricted stock unit owned by the reporting person was cancelled and converted into the right to receive $40.75 (the &quot;Per Share Price&quot;) less any required withholding taxes.</footnote>
        <footnote id="F2">The reporting person will receive an additional amount in cash equivalent to 4.97 shares due to dividend equivalents.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Lauren Neiswender, Power of Attorney</signatureName>
        <signatureDate>2017-02-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
