Historical Rules · 1 October 2009
Rule 509.7
Official source text from the 1 October 2009 Rules release
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), then the active player announces the new creature's placement in the blocked creature's damage assignment order. The relative order among the remaining blocking creatures is unchanged. A creature put onto the battlefield this way is "blocking" but, for the purposes of trigger events and effects, it never "blocked." Example: Lumengrid Warden is blocked by Runeclaw Bear. The defending player casts Flash Foliage, which puts a Saproling token onto the battlefield blocking the Lumengrid Warden. Lumengrid Warden's controller announces the Lumengrid Warden's damage assignment order as the Saproling token, then Runeclaw Bear.