My 98 Blazer's heat only comes out of the top of the dash no matter which setting is on the selector. Any ideas? Thanks!

Take out the panel where all the switches are and see if something got disconnected. There might be a plastic piece that moves the vents from the windshield to the front vents that's broken. My Breeze did the same thing all winter then in the summer I turned on the a/c and now those damn front vents work again!

Automatic or manual ac/heater controls?
Got good engine vacuum from engine to it?

I would agree with MT sounds like a vacuum issue.

Manual controls. I'll check for vacuum. Do the shutters run off vacuum? Thanks!

Yes a lot of it is vacuum controlled.
Check vacuum line at engine and vacuum tank.
MT