There are a number of uses for the word blank:

• A blank (archeology) is a thick, shaped stone biface of suitable size and configuration for refining into a stone tool.
• A blank (Scrabble) is a unique tile that rather than having one of the letters of the alphabet printed on it, it has a blank face and can be substituted for any letter the player desires.
• A blank (cartridge) is a type of cartridge for a gun that contains gunpowder but no bullet or shot.
• A blank expression is either one of slight confusion when someone refers to something which you do not understand or a face where you which to hide your expression from other people.

Also: blank as in 'unused' or 'clear'; a blank slate, a key blank (an uncut key), a blank sheet of paper. etc.