An ability that reads “Whenever [a 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.
September 30, 2011 · Innistrad
During the declare blockers step, if an attacking creature is removed from combat or a spell or ability causes it to stop being blocked by a blocking creature, the attacking creature is removed from all relevant damage assignment orders. The relative order among the remaining attacking creatures is unchanged.
June 17, 2011 · Commander 2011
During the declare blockers step, if an attacking creature is removed from combat or a spell
February 4, 2011 · Mirrodin Besieged
During the declare blockers step, if an attacking creature is removed from combat or a spell or ability causes it to stop being blocked by a blocking creature, the attacking creature is removed from all relevant damage assignment orders. The relative order among the remaining attacking creatures is unchanged.
October 1, 2010 · Scars of Mirrodin
During the declare blockers step, if an attacking creature is removed from combat or a spell
February 1, 2010 · Worldwake
During the declare blockers step, if an attacking creature is removed from combat or a spell or ability causes it to stop being blocked by a blocking creature, the attacking creature is removed from all relevant damage assignment orders. The relative order among the remaining attacking creatures is unchanged.