My blast oven, and I assume most blast ovens, has a hole in the top where it blasts air out of the oven. It does have a sliding cover though if for some reason you wanted to block it off. I leave mine mostly open, so there is no moisture buildup inside it.
The main downside to it is whatever humidity is in the ambient air. If you’ve already nearly dried your filament to death inside the blast oven, but a rain system arrives and starts raising the humidity of the ambient air, then your spool will actually gain some moisture weight, even though it is still inside a heated blast oven.
I should hasten add that the same would happen with most 3D filament dryers also. The solution is to pump dry air into the blast oven, which at some point I am going to do so that I can have fully repeatable consistency.