What is In Blank?

Legal Definition
This is generally applied to endorsements, as, endorsements in blank, which is one not restricted, made by the endorser simply writing his name. See Endorsement.
-- Bouviers Law Dictionary
Legal Definition
A term applied to the endorsement of a bill or note where it consists merely of the endorser's name, without restriction to any particular endorsee. 2 Steph. Comm. 164.
-- Black's Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary