I have a 2000 Blazer with manual heater controls. When it finally warms up and driven on the highway the heat will blast. When idling at a light the heat starts getting cool. Any ideas? I was thinking thermostat but it does get hot eventually but just doesn't stay hot at lower idle speed.The positions all seem to work fine for air flow.

Sounds like your cooling system is some sludged up.
You can take both heater hoses going to your heater loose, apply garden hosewater to one then the other for a minute or so to flush the heater core. Do NOT pinch the hoses or allow any pressure to build up for any reason....you want to flush one line with the other wide open then swap.

Check the coolant level, sounds to me like it's low.