If the effect that puts a creature onto the battlefield blocking specifies it’s blocking a certain creature and that creature is no longer attacking, the creature is put onto the battlefield but is never considered a blocking creature. The same is true if the controller of the creature that’s put onto the battlefield blocking isn’t a defending player for the specified attacking creature.
May 3, 2019 · War of the Spark
Any abilities that triggered on blockers being declared or that triggered during the process described in rules 509.1–3 are put onto the stack before the active player gets priority; the order in which they triggered doesn’t matter. (See rule 603, “Handling Triggered Abilities.”)
January 25, 2019 · Ravnica Allegiance
Any abilities that triggered on blockers being declared or that triggered during the process described in rules 509.1-3 are put onto the stack before the active player gets priority; the order in which they triggered doesn't matter. (See rule 603, "Handling Triggered Abilities.")
September 30, 2016 · Kaladesh
Any abilities that triggered on blockers being declared or that triggered during the process described in rules 509.1–3 are put onto the stack before the active player gets priority; the order in which they triggered doesn’t matter. (See rule 603, “Handling Triggered Abilities.”)
May 1, 2014 · Journey into Nyx
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.
February 1, 2014 · Born of the Gods
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.
September 30, 2011 · Innistrad
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.
June 17, 2011 · Commander 2011
An ability that reads “Whenever [this creature] blocks, . . .” generally triggers only once
April 1, 2011 · Duel Decks: Knights vs. Dragons
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.
February 4, 2011 · Mirrodin Besieged
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.
October 1, 2010 · Scars of Mirrodin
An ability that reads “Whenever [this creature] blocks, . . .” generally triggers only once
February 1, 2010 · Worldwake
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.