Does reach allow a creature to block a flying creature even if the blocking creature doesn't have flying itself?

Reach is a keyword ability defined in CR 702.17a: 'A creature with reach can block creatures with flying.' This is a straightforward exception to the normal evasion rule for flying, which states that creatures with flying can only be blocked by other creatures with flying or reach (CR 702.9b).

So a creature like Spider (with reach but no flying) can legally declare itself as a blocker against an attacking creature with flying. It does not need flying itself — reach alone is sufficient to 'reach up' and intercept aerial attackers.

During the declare blockers step (CR 509.1), the game checks that each blocking creature is legally able to block the attacker. A creature with only reach passes this check against a flying attacker, because reach satisfies the flying evasion requirement.

Concrete example: Your opponent attacks with a Snapping Drake (2/2, flying). You control a Giant Spider (2/4, reach). Even though Giant Spider has no flying, you may declare it as a blocker for Snapping Drake. The Drake is blocked and, unless it has trample or another ability, deals its damage only to the Spider.

Note that reach does not grant flying — a creature with only reach cannot itself fly over blockers. It solely enables the creature to block flyers (CR 702.17a).