What is Blank?

Legal Definition
A space left in writing to be filled, up with one or more words, in order to make sense. 1. In what cases the ambiguity occasioned by blanks not filled before execution of the writing may be explained 2. in what cases it cannot be explained.

2. - 1. When a blank is left in a written agreement which need not -have been reduced to writing, and would have been equally binding whether written or unwritten, it is presumed, in an action for the non-performance of the contract, parol evidence might be admitted to explain the blank. And where a written instrument, which was made professedly to record a fact, is produced as evidence of that fact which it purports to record, and a blank appears in a material part, the omission may be supplied by other proof. 1 Phil. Ev. 475 1 Wils. 215; 7 Verm. R. 522; 6 Verm. R. 411. Hence a blank left in an award for a name, was allowed to be supplied by parol proof. 2 Dall. 180. But where a creditor signs a deed of composition leaving the amount of his debt in blank, he binds himself to all existing debts. 1 B. & A. 101; S. C. 2 Stark. R. 195.

3. - 2. If a blank is left in a policy of insurance for the name of the place of destination of a ship, it will avoid the policy. Molloy, b. 2, c. 7, s. 14; Park, Ins. 22; Wesk. Ins. 42. A paper signed and sealed in blank, with verbal authority to. fill it up, which is afterwards done, is void, unless afterwards delivered or acknowledged and adopted. 1 Yerg. 69, 149; 1 Hill, 267 2 N. & M. 125; 2 Brock. 64; 2 Dev. 379 1 Ham. 368; 6 Gill & John. 250; but see contra, 17 S. & R. 438. Lines ought to be drawn wherever there are blanks, to prevent anything from being inserted afterwards. 2 Valin's Comm. 151.

4. When the filling up blanks after the execution of deeds and other writings will vitiate them or not, see 3 Vin. Abr. 268; Moore, 547; Cro. Eliz. 626; 1 Vent. 185; 2 Lev. 35; 2 Ch. R. 187; 1 Anst. 228; 5 Mass. 538; 4 Binn. 1; 9 Crancb, 28; Yelv. 96; 2 Show. 161; 1 Saund. Pl. & Ev. 77; 4 B. & A. 672; Com. Dig. Fait, F 1; 4 Bing. 123; 2 Hill. Ab. c. 25, §80; n. 33, §54-and 72; 1 Ohio, R. 368; 4 Binn. R. 1; 6 Cowen, 118; Wright, 176.
-- Bouviers Law Dictionary
Legal Definition
A space left unfilled in a written document, in which one or more words or marks are to be inserted to complete the sense. Angle v. Insurance Co., 92 U. S. 337, 23 L. Ed. 556. Also a skeleton or printed form for any legal document, in which the necessary and invariable words are printed in their proper order, with blank spaces left for the insertion of such names, dates, figures, additional clauses, etc., as may be necessary to adapt the instrument to the particular case and to the design of the party using it.
See also
-- Black's Law Dictionary
Legal Definition
A space in a document intended to be filled to complete the meaning.
-- Ballentine's Law Dictionary