In community card poker, refers to holding one of the opponent's outs, typically when the board threatens a straight or straight draw. A blocker is also having a combination of cards that turn your opponents outs into your own, such as having four to a straight flush. The two cards to give you a straight flush are blockers against his high flush draw. The board was A23 but with my pair of fives I held two blockers to the straight. Compare to "dry ace".