Ok I just got my blazer a month ago and I had the AC fixed by the guys I bought it from well now that its getting cold out my heat doesn't work and I have flushed my system. My buddy said it was the blend air door and if that is so are those hard to change?

 

the blend air door is actually a butterfly valve and they are behind the dash controls behind the dash what i would do is tear it down and clean the valve it may be sticking one way.

 

The blend doors don't normally stick, but rather the vacuum actuator that controls them may. Are all of the other functions working? Do you have auto climate control or manual?