Historical Rules · 23 January 2015
Rule 509.4b
Official source text from the 23 January 2015 Rules release
An ability that reads “Whenever [this creature] blocks a creature, . . .” triggers once for each attacking creature the creature with the ability blocks. It triggers if the creature is declared as a blocker. It will also trigger if an effect causes that creature to block an attacking creature, but only if it wasn’t already blocking that attacking creature at that time. It won’t trigger if the creature is put onto the battlefield blocking.