Blanks in poker come across to a poker player just like the bullets we have which are used in guns but without a significant effect when shot. A blank is a card that when played, will do little to help a player by being less in value. They are the cards that will simply fill in the gaps in time and space during a game. When poking fun at an opponent, a player can say something like this "I was certain that he had some kind of draw so I thought I should bet him with his river of blanks."
A blank is essentially that card or group of cards which can't affect the outcome of a hand. Out of suspicion that an opponent is holding a deck of blanks, a poker player can decide to make an educated or well informed guess by betting again with the other person. In such a situation, s/he is sure that nothing significant will come out of the game that will favor the other player which will give him or her the edge of winning.