96 Chevy Blazer S-10 88,000 miles

I recently had issues with my heater not getting warm. I have replaced water pump due to it leaking but since then, now my heater blower works fine but doesn't blow warm air. I have also replaced thermostat since this problem started but still didn't fix it. Does anyone know where I need to go next step as to fixing the problem? Also, my heater core seems to be getting warm but doesn't leak in any way. Thanks