Try the heater valve.
The most common problem causing a 2000 Blazer to stay on heat is a problem with the blend door. Check the blend door and its control circuit to fix the problem.
I have a 2000 daewoo nubria and i get this problem sometimes...if its just that nothing is coming out of the vents but you have the heat or ac on pop the hood and open it then let it slam shut mine usually starts right up after that.
Winter Heat Series was created in 1994.
The Heat Is On - TV series - was created in 1990.
Had the same problem on my 2000. Heat/AC and fan was unresponsive, with the small amber lights only coming on very dim. Ended up being the ignition switch.
Heat - TV series - was created in 2008-07.
Thermostat stuck open. Replace! A low coolant level will also cause this problem. The blend door could be stuck also.
Have the same problem with a 635/csi. I'm not technically a mechanic, but, pretty sure it's the heater control valve.
My daughter had the same trouble with her 2000 escort. It was low on coolant. 2nd opinion: The most likely problem is the thermostat, which as I have read in here is a pain to get to.
IF YOUR CAR LEAKS WATER AND HEAT BLOWS COLD AIR WHAT THE PROBLEM
heat
The duration of L.A. Heat - TV series - is 3600.0 seconds.