I changed the thermostat and the engine temp came up but the heater still isn't as hot as "I" think it should be. Thanks for all the replies. The thermostat was really rust-red in color, I'm told that's because of dex-cool. Lines coming out of core are both warm, one more so than the other. I was told that 20 degrees difference indicates a clogged core.

 

Dexcool is clear orange in color, but if it is not flushed often enough, (3 years/36K miles) or green coolant is added to it, it will turn a rusty sludge color and clog up the entire cooling system. Generally, if you can feel a difference between the two hoses with your bare hand at the core, the core is clogged. Register outlet temp should be no lower than 130F.

 

You should consider a cooling system pressure test. On a 4.3L, the top two reasons for a coolant temperature below 190 after warm up is a failing lower intake manifold gasket and / or a water pump. The 4.3L runs funny on the Blazer as far as cooling goes. Failing water pumps usually increase temps, in the blazer at the beginning stage it often runs cooler.

Run a pressure test and I bet you will find you answer.

 

I had the same issue... got one of those prestone flush kits. Flushed it twice and put in a hotter thermostat. It is a lot better now. I was shocked at the level of muddy crud that came out of there, even with just the garden hose flush.