Historical Rules · 1 October 2009
Rule 509.4f
Official source text from the 1 October 2009 Rules release
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. 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 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.