text: Each of those creatures blocks only if it's able to do so as the declare blockers step begins. If, at that time, one of the creatures is tapped, is affected by a spell or ability that says it can't block, or no creatures are attacking that creature's controller or a planeswalker controlled by that player, then it doesn't block. If there's a cost associated with having the creature block, the player isn't forced to pay that cost, so it doesn't block in that case either.