Flying grants evasion, meaning a creature with flying can only be blocked by creatures with flying or reach (CR 702.9b). However, this restriction governs which blockers are legal, not whether blocking is required.
The rule about blocking is separate: during the declare blockers step, if an attacking creature is unblocked and the defending player has at least one creature that could legally block it, they are not forced to block — blocking is always optional (CR 509.1). A player is never compelled to block unless a specific card effect says so (e.g., 'must block if able').
So the key point is: flying does not create a forced-block requirement. The defending player may block with their flying creature, but they are not required to, even if it is the only legal blocker available.
Example: Your opponent attacks with a 2/2 Flying Drake. You have only one creature, a 1/1 Pegasus with flying. You are allowed to block the Drake with your Pegasus, but you are not forced to. You may choose to take 2 damage and keep your Pegasus alive.
The only exception would be if a card effect explicitly states that a creature 'must block if able,' in which case the controller would be required to assign it as a blocker if it is a legal one (CR 509.1c, 702.2).
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.