I have a 96 blazer that will not blow air thru the dash vents, but blows air thru defroster and floor vents fine. Any idea what the problem might be?

Relay on the firewall. I beleive its the middle one. Its black. $18 at autozone. I just replaced mine.

Is this a blower relay?

Yes...There are 3 relays there. They are all the same.

If the air flow is not switching always check for good engine vacuum from engine to ac controls.

Broken plastic line vacuum or tank unpluged or engine vacuum line unplugged.

I replaced the relay, but it did not fix the problem. Still no air blowing thru dash vent.

It Is Not ectrical.
It Is Vacuum Problem.