If a blocking creature is given flying after blockers are declared does it still block?

Once blockers are declared in the Declare Blockers Step, the blocking relationship is locked in. Gaining or losing abilities — including flying — after that point does not remove a creature from combat or change its blocking status. This is covered by CR 509.5, which states that a creature remains blocking even if it loses relevant abilities.

Flying (and similar evasion abilities like shadow or fear) only matter at the moment blockers are declared (CR 509.1). During that declaration, a creature without flying cannot legally block a flying attacker unless another ability permits it. But once the declaration is complete and the step moves forward, the game no longer checks those requirements.

This means abilities that grant evasion or remove evasion mid-combat generally don't retroactively undo a legal block. The block stands, damage will be assigned normally, and both creatures may deal damage to each other in the Combat Damage Step (CR 510.1).

Concrete example: Your opponent's 2/2 Bird (flying) attacks. You block it with your 2/2 Bear (no flying). In response to nothing in particular, someone casts a spell giving your Bear flying. The Bear already legally blocked — the Bird is still blocked, and both creatures will deal 2 damage to each other when combat damage is assigned.

Conversely, if the Bear had not yet been declared as a blocker, gaining flying would then make it eligible to block the Bird — but that's a separate situation governed by the legality check at declaration time (CR 509.1b).