309.4.

Abilities that trigger on a creature becoming blocked trigger only at the first point the creature becomes blocked that combat. They will trigger if a creature becomes blocked by a creature declared as a blocker, by a creature that's put into play as a blocker, or by an effect, but only if the attacking creature hadn't yet been blocked that combat. They will not trigger if a creature becomes blocked, and then the blocking creature's characteristics change to match the ability's trigger condition.

Example: A creature has the ability "Whenever this creature becomes blocked by a white creature, destroy that creature at end of combat." If that creature is blocked by a black creature that is later turned white, the ability will not trigger.