Historical Rules · 1 October 2009
Rule 509.4d
Official source text from the 1 October 2009 Rules release
An ability that reads "Whenever [this creature] becomes blocked by a creature, . . ." triggers once for each creature that blocks the named creature. It will also trigger if an effect causes a creature to block the attacking creature, even if it had already been blocked that combat. It won't trigger if the creature becomes blocked by an effect rather than a creature.