The Collaborative International Dictionary of English v.0.48:

Blank \Blank\, n.
   1. Any void space; a void space on paper, or in any written
      instrument; an interval void of consciousness, action,
      result, etc; a void.
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            I can not write a paper full, I used to do; and yet
            I will not forgive a blank of half an inch from you.
                                                  --Swift.
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            From this time there ensues a long blank in the
            history of French legislation.        --Hallam.
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            I was ill. I can't tell how long -- it was a blank.
                                                  --G. Eliot.
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   2. A lot by which nothing is gained; a ticket in a lottery on
      which no prize is indicated.
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            In Fortune's lottery lies
            A heap of blanks, like this, for one small prize.
                                                  --Dryden.
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   3. A paper unwritten; a paper without marks or characters a
      blank ballot; -- especially, a paper on which are to be
      inserted designated items of information, for which spaces
      are left vacant; a bland form.
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            The freemen signified their approbation by an
            inscribed vote, and their dissent by a blank.
                                                  --Palfrey.
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   4. A paper containing the substance of a legal instrument, as
      a deed, release, writ, or execution, with spaces left to
      be filled with names, date, descriptions, etc.
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   5. The point aimed at in a target, marked with a white spot;
      hence, the object to which anything is directed.
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            Let me still remain
            The true blank of thine eye.          --Shak.
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   6. Aim; shot; range. [Obs.]
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            I have stood . . . within the blank of his
            displeasure
            For my free speech.                   --Shak.
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   7. A kind of base silver money, first coined in England by
      Henry V., and worth about 8 pence; also, a French coin of
      the seventeenth century, worth about 4 pence. --Nares.
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   8. (Mech.) A piece of metal prepared to be made into
      something by a further operation, as a coin, screw, nuts.
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   9. (Dominoes) A piece or division of a piece, without spots;
      as, the "double blank"; the "six blank."
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   In blank, with an essential portion to be supplied by
      another; as, to make out a check in blank.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

IN BLANK. This is generally applied to indorsements, as, indorsements in
blank, which is one not restricted, made by the indorser simply writing his
name. See Indorsement.