509.3d

An ability that reads “Whenever [a creature] becomes blocked by a creature, . . .” triggers once for each creature that blocks the specified creature. It triggers if a creature is declared as a blocker for the attacking creature. It will also trigger if an effect causes a creature to block the attacking creature, but only if it wasn’t already blocking that attacking creature at that time. In addition, it will trigger if a creature is put onto the battlefield blocking that creature. It won’t trigger if the creature becomes blocked by an effect rather than a creature.