A creature with reach is explicitly able to block creatures with flying, as stated in CR 702.17a: "Reach" means "This creature can block creatures with flying." Flying normally restricts which creatures can block it (only creatures with flying or reach), per CR 702.9b.
Trample is a completely separate ability that governs how excess combat damage is assigned — specifically, the attacking creature can assign damage beyond what is needed to destroy blockers to the defending player or planeswalker (CR 702.19b). Trample has no interaction with what creatures are legally able to block.
So the two questions — "Can this creature be blocked?" and "How does this creature deal damage?" — are answered independently. A creature with flying AND trample can be blocked by a creature with reach, and then trample simply determines how the attacker assigns its combat damage.
Example: Your opponent attacks with a 5/5 Dragon that has flying and trample. You block with your 2/3 Spider that has reach. The Spider can legally block the Dragon because of reach. When combat damage is assigned, the Dragon must assign at least 2 damage to the Spider (lethal damage), and can then assign the remaining 3 damage as trample damage to your opponent.
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.