Historical Rules · 10 June 2022
Rule 509.5b
Official source text from the 10 June 2022 Rules release
An ability that reads “Whenever [a 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.