A blank is any card that comes that seems to not matter or affect anyone's range significantly. It usually refers to turn and river cards, but could be used for flopaction as well. A good example of this is a board of AsKsQs and then a 2c comes up. Often, this 2c wouldnt help anyone and could be referred to as a blank.
Example: He 3b me on the flop, and I smooth called. The turn was a blank and he shoved all in.