You have a 2000 Blazer After installing the water pump the heater just blows cold air you have replaced the thermostat back flush the cooling system Both hoses that goes into the heater core are hot?

Answer 1

vacuum operated valve in heater hose not opening

Addition:

Another possibility is that the heater core is plugged. The Dex cool coolant is notorious for clumping up and clogging up the heater core when exposed to air for a while (air pockets in the system after adding in coolant)

Disconnect the 2 hoses on the firewall that run to the engine (heater core inlet and outlet). Take 2 pieces of automotive hose or whatever you can get your hands on the make a snug fit around the pipes coming out from the fire wall. run a garden hose on the right hose (outlet). Don't push the garden hose directly to the heater core. you can damage it. heater cores can handle pressure 14-16PSI on average, a garden hose can do 40-70PSI. you risk damaging it. Run the water into the hose and keep an eye out on the other one and see if coolant is flowing, you can push the garden hose closer to the outlet hose on the heater core if needed, just be careful. once you're getting clean water all the way through and no chunks or pieces are coming out. Reconnect the hoses from the engine and start it up. You'll have to add some more coolant to make up for what was flushed out.

If both heater hoses were hot chances are the heater is not plugged and there is no vacuum operated valve in the heater hoses on that model. My guess would be the temperature blend door actuator behind the dash has failed.