If a creature is put onto the battlefield blocking, its controller chooses which attacking creature it’s blocking as it enters the battlefield (unless the effect that put it onto the battlefield specifies what it’s blocking). A creature put onto the battlefield this way is “blocking” but, for the purposes of trigger events and effects, it never “blocked.”
July 12, 2019 · Core Set 2020
Fourth, the active player gets priority. (See rule 117, “Timing and Priority.”)
May 3, 2019 · War of the Spark
Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)
January 25, 2019 · Ravnica Allegiance
Fourth, the active player gets priority. (See rule 116, "Timing and Priority.")
September 30, 2016 · Kaladesh
Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)
May 1, 2014 · Journey into Nyx
Fourth, any abilities that triggered on blockers being declared go on the stack. (See rule 603, “Handling Triggered Abilities.”)
September 30, 2011 · Innistrad
Fourth, any abilities that triggered on blockers being declared go on the stack. (See rule 603, "Handling Triggered Abilities.")
June 17, 2011 · Commander 2011
Fourth, any abilities that triggered on blockers being declared go on the stack. (See rule 603,
April 1, 2011 · Duel Decks: Knights vs. Dragons
Fourth, any abilities that triggered on blockers being declared go on the stack. (See rule 603, "Handling Triggered Abilities.")
February 4, 2011 · Mirrodin Besieged
Fourth, any abilities that triggered on blockers being declared go on the stack. (See rule 603, “Handling Triggered Abilities.”)
October 1, 2010 · Scars of Mirrodin
Fourth, any abilities that triggered on blockers being declared go on the stack. (See rule 603,
February 1, 2010 · Worldwake
Fourth, any abilities that triggered on blockers being declared go on the stack. (See rule 603, "Handling Triggered Abilities.")