The blank poker term is commonly used when referring to a community card that apparently doesn’t affect the strength of any player’s hand.
For example, you have an AsQh, and the flop comes 4s, 6s, and an Ah. The fourth street produces a Qs, giving you two pairs and opening up the possibility of a flush.
As the river comes, it brings a 9c. The river card is a blank as it doesn’t help you in any way and most likely doesn’t help your opponent as it doesn’t complete any draws.