Summoning sickness (the colloquial term for the rule in CR 302.6) restricts a creature from attacking and from activating abilities that include the tap symbol (T) or untap symbol (Q) in their cost. It does not restrict a creature from blocking.
CR 302.6 states that a creature cannot attack unless it has been under its controller's control since the beginning of that player's most recent turn. Blocking is governed separately by CR 509.1, which simply requires the creature to be untapped and have no abilities that prevent it from blocking — summoning sickness is not one of those restrictions.
This means a creature that entered the battlefield this turn is perfectly legal to declare as a blocker during the opponent's combat phase, even though it cannot be declared as an attacker on your own turn yet.
Example: You cast Grizzly Bears on your turn. Your opponent attacks you with a 3/3 on their very next turn. You may legally declare Grizzly Bears as a blocker, even though it has summoning sickness and could not have attacked on your turn.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.