Historical Rules · 25 August 2017
Rule 810.7d
Official source text from the 25 August 2017 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.