Defender is a static ability defined in CR 702.3b: "A creature with defender can't attack." That's the entire effect of the ability — it restricts attacking only, and says nothing whatsoever about blocking.
Blocking is governed by CR 509.1, which lists the conditions that make a creature an illegal blocker (e.g., being tapped, having a restriction like "can't block," or being affected by evasion abilities on the attacker). Having defender is not on that list, so it does not prevent a creature from being declared as a blocker.
This is a very common misconception, likely because defender creatures are often depicted as walls or fortifications that feel "defensive." The flavor suggests staying put, but the rules only penalize them for attacking, not for blocking.
Concrete example: Your opponent attacks with a 3/3 creature. You control a Wall of Omens (0/4 with defender). You may legally declare Wall of Omens as a blocker. It will absorb up to 4 damage from the attacker, functioning exactly as any other blocker would.
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.