If your temp gage is reading normal it's not likely a pressure issue or coolant level issue.The design of the 2000 Blazer heater core is horrible. It tends to trap any air in the system at the top of the core because the inlet and outlet are at the bottom of the core. Sometimes you can flush it multiple times with out removing the air. Are you hearing any gurgling noise when revving the engine or accelerating while driving? That would indicate the presence of air in the core.

 

 

I have the best luck jacking up the passenger side to make the fill neck on the radiator the highest point.

Then starting the engine and letting it come up to temp with the cap off and adding water as needed.

I have put in two Radiators in the last year on two trucks and done the LIM as well

 

 

I am assuming the gauge is in Celsius so you should be at about 88-95 degree C. Are there aftermarket electric fans? and is the Thermostat installed?

In the US it would throw a code for not reaching operating temperature for emissions reasons.