Historical Rules · 1 April 2011
Rule 509.4a
Official source text from the 1 April 2011 Rules release
An ability that reads "Whenever [this creature] blocks, . . ." generally triggers only once each combat for that creature, even if it blocks multiple creatures. It triggers if the creature is declared as a blocker. It will also trigger if that creature becomes a blocker as the result of an effect, but only if it wasn't a blocking creature at that time. (See rule 509.1g.) It won't trigger if the creature is put onto the battlefield blocking.