[N8SBlaze]

Any idea if the water pump has ever been changed on the Blazer? Just a thought.
The gurgling sound behind the dash is probably air trapped in the heater core. I have the same sound on mine from time to time. I have seen a procedure posted on this forum to get the air out by parking on an incline, or put the front wheels on ramps to raise the front end. When the engine is cold remove the rad cap, start the vehicle, let the coolant circulate, as the air works its way out of the system it will burp at the rad cap opening, add more coolant as needed and replace the rad cap. This works because on level ground the heater core is positioned higher than the radiator, and air can get trapped in the high spots. When the front end is inclined the radiator is then higher than the heater core.