Can you block with a creature that became tapped after the declare blockers step?

Blocking eligibility is checked at one specific moment: when blockers are declared during the Declare Blockers Step (CR 509.1). At that point, a creature must be untapped to be a legal blocker. Once a creature is successfully declared as a blocker, it remains a blocker for the rest of combat regardless of what happens to it afterward.

This means the question has two distinct cases. If a creature is tapped before or during the declare blockers step, it cannot block (CR 509.1a). If a creature already declared as a blocker becomes tapped afterward — say, by an opponent's instant — it remains a blocker and will still deal and receive combat damage normally (CR 509.2).

The confusion often arises because players think tapping a declared blocker removes it from combat. It does not. Tapping only prevents a creature from blocking if it happens before the blocker is legally declared.

Concrete example: Your 2/2 Bear successfully blocks your opponent's 3/3 Centaur. Your opponent then casts Twiddle targeting your Bear, tapping it. The Bear is now tapped, but it is still a legal blocker — it will still deal 2 damage to the Centaur and receive 3 damage when combat damage is dealt.

Key rules: CR 509.1a requires blockers to be untapped at declaration. CR 509.2 confirms that after blockers are declared, each chosen creature becomes a blocking creature and combat proceeds. A creature's tapped status after declaration does not remove it from combat.