Historical Rules · 16 July 2010
Rule 802.4
Official source text from the 16 July 2010 Rules release
If more than one player is being attacked or controls a planeswalker that's being attacked, each defending player in APNAP order declares blockers as the declare blockers step begins. (See rule 101.4 and rule 509, "Declare Blockers Step.") The first defending player declares all his or her blocks, then the second defending player, and so on.