Does a creature become a legal blocker again after it loses flying mid-combat if it was already declared as a blocker?

When blockers are declared during the Declare Blockers Step, the game checks legality of each blocker at the moment of declaration (CR 509.1). If a creature with flying legally blocked a flying attacker, that block is established and recorded by the game.

After blockers are declared, losing an ability like flying does not retroactively invalidate the block. CR 509.2 confirms that a creature remains a blocking creature even if it is removed from combat only under specific circumstances listed in the rules — losing an evasion ability is not one of them. The block simply stays in place.

This is because evasion abilities like flying (CR 702.9) only function during the declaration of blockers to restrict which creatures can block. Once that window has passed, they have no further mechanical effect on an already-established block.

Example: Your opponent attacks with a 2/2 Bird (flying). You block with your 2/2 Angel (flying). In response to nothing, someone casts an instant that makes your Angel lose flying. The block is already declared and legal — the Angel still blocks the Bird, and both deal damage normally in the Damage Step.

The key takeaway: legality of blocking is checked only at declaration (CR 509.1b). Mid-combat ability loss cannot undo an already-legal block assignment.