<SUBMISSION>
<ACCESSION-NUMBER>0001091171-17-000034
<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>MEADS MINDY C
<CIK>0001245042
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-50763
<FILM-NUMBER>17622412
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>3333 BEVERLY ROAD
<CITY>HOFFMAN ESTATES
<STATE>IL
<ZIP>60179
</MAIL-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_148737012009272.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>0001245042</rptOwnerCik>
            <rptOwnerName>MEADS MINDY C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O BLUE NILE, INC. 411 FIRST AVENUE S</rptOwnerStreet1>
            <rptOwnerStreet2>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>12197</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>35.0</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2017-02-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4051</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2023-05-21</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4051.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </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">Pursuant to the Merger Agreement, at the Effective Time, each outstanding stock option owned by the reporting person that has an exercise price per share of common stock underlying the stock option that is less than the Per Share Price will be cancelled and converted into the right to receive an amount in cash, less any required withholding taxes, determined by multiplying (a) the excess of the Per Share Price over the exercise price of such stock option by (b) the number of shares of common stock underlying the stock option.</footnote>
    </footnotes>

    <remarks></remarks>

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