Ok, the first thing I would recommend is to verify that both of your heater hoses back at the firewall are getting hot. This would show if you are getting a good amount of flow through the heater core. The blazers have been notorious for having a build up from the dexcool in the heater core causing for this problem. I see that you did flush it, so I would figure that you have seen if the heater core does have a decent amount of flow through it. I just wanted to mention this as a possibility.
The next common failure if the actual electrical actuator that control the temperature door in the heater box. This is a simple electric actuator that can fail and cause the problems you are having. We can do some tests to check this problem, but you will need to remove the complete dash assembly as this actuator is on top of the heater box and cannot be accessed from underneath. I would also check the fuse labeled HVAC 1 in the instrument panel fuse box. There is also a chance of this being a problem with the actual heater a/c control unit in the dash, but we need to do some testing before we can come to that conclusion. Feel free to contact me with any questions, findings or if you would like to get into the testing. You will need a voltmeter to perform these tests.