The term “blank” in poker is used to describe a card that doesn’t influence the action as it is highly unlikely to improve any of the players’ hands or influence their perceived ranges.
Almost every card in a deck can be a blank, depending on the situation.
For example, after a lot of action on a board of 4-5-7, a Q on the turn is considered a blank. Given the information available, this card is highly unlikely to influence the player’s ranges.
Whether a card is blank or not is determined solely based on the action leading up to it. You shouldn’t be too quick to judge a card as blank before carefully considering the action before the flop, on the flop, or on the turn.
If a card is truly a blank, it is not a good idea to start bluffing on it, either. Your opponent will likely be aware of the fact that the card changes nothing, and the story you try to tell will not add up.