Historical Rules · 1 February 2010
Rule 806.8d
Official source text from the 1 February 2010 Rules release
As the declare blockers step begins, the defending team declares blockers. Creatures controlled by the defending players can block any attacking creatures. The defending team has one combined block, and that set of blocking creatures must be legal as a whole. See rule 509.1. Example: If an attacking creature has forestwalk and either player on the defending team controls a Forest, the creature can't be blocked.