Rule 509.3f

If an ability triggers when a creature with certain characteristics blocks, it will trigger only if the creature has those characteristics at the point blockers are declared, or at the point an effect causes it to block. If an ability triggers when a creature with certain characteristics becomes blocked, it will trigger only if the creature has those characteristics at the point it becomes a blocked creature. If an ability triggers when a creature becomes blocked by a creature with certain characteristics, it will trigger only if the latter creature has those characteristics at the point it becomes a blocking creature. None of those abilities will trigger if the relevant creature’s characteristics change to match the ability’s trigger condition later on. Example: A creature has the ability “Whenever this creature becomes blocked by a white creature, destroy that creature at end of combat.” If the creature becomes blocked by a black creature that is later turned white, the ability will not trigger.