Does a creature with reach block a creature with flying that was given flying after attackers were declared?

A creature with reach can block creatures with flying (CR 702.17a). The key timing question is: does the attacking creature have flying when blockers are declared (the declare blockers step, CR 509)? If yes, reach is sufficient to block it.

Flying and reach interact at the moment of blocker declaration (CR 509.1b). The rules check whether the blocking creature is "able to block" the attacker at that moment. If the attacker already has flying when you declare blockers, a creature with reach can legally block it, no matter when during the turn flying was granted.

It does not matter that flying was added after attackers were declared (CR 508). The game only checks blocking restrictions at the time blockers are declared, not retroactively based on what the creature looked like during the declare attackers step.

Example: Your opponent attacks with a 2/2 Grizzly Bears (no flying). In response to nothing — or during the combat phase before blockers — they cast Levitation's effect or use a pump spell to give it flying. When the declare blockers step begins, the Bears now have flying. Your 1/4 creature with reach can legally block it because reach permits blocking flying creatures at that moment.

Note: if a creature loses flying after it has been legally declared as blocked, the block remains valid — blocks are only checked for legality at declaration (CR 509.1b, 702.17a).