What is Blanc Seign?

Legal Definition
In Louisiana, a paper signed at the bottom by him who intends to bind himself, give acquittance or compromise, at the discretion of the person whom he intrusts with such blanc seign, giving him power to fill it with what he may think proper, according to agreement. Musson v. U. S. Bank, 6 Mart. O. S. (Lat) 718.
-- Black's Law Dictionary
Legal Definition
A blank signed by the principal intended to be filled out as a power of attorney by the appointee. See 6 Mart. (O. S.) (La.) 707.
-- Ballentine's Law Dictionary