Did you replace the gas cap?
Gas cap and purge valve are the 2 most likely faults. Could also be a damaged or loose hose in the evap system possibly at the vapor canister.
You'll need to have a smoke test done. Some EVAP leaks can be so small you can't see it with the naked eye. HTH. -Jim
Looking for a Used TrailBlazer in your area?
CarGurus has 4,277 nationwide TrailBlazer listings starting at $1,500.
Search Chevrolet TrailBlazer Questions
Chevrolet TrailBlazer Experts