I went through the same thing with my Blazer. I replaced the thermostat and flushed the system and I still wouldn't get heat. The problem ended up being the waterpump. It was efficient enough to keep the engine cool, but couldn't flow enough to circulate through the heater core. Now my heat is super hot and the engine still runs at perfect operating temps. On a side note, I jacked the front passenger side of the blazer up as high as I could to fill the radiator that way the heater core is lower than the fill point. After I filled it, I drove for a few miles, brought it back and jacked it back up to let it cool. After about an hour, any air that was in there, worked itself out since it was jacked up and when it cooled, I topped it off again with a 50/50 mix. Good luck.