Heater blowing cold air

Chevy Camaro 1996 101,000 miles 3.8L; This past week the heater started blowing only cold air. The temperature guage behaves like normal. Warms up to around 190. Gets a little warmer during stop and go traffic, then quickly back to normal once moving along again, so I think the thermostat is working ok. I checked around the heater core and I can definitely tell changing the temperature setting opens and closes the vent door and the fan is blowing. The hoses going to and from the heater core are both cold when checked near the water pump. The radiator is full. I replaced the water pump around 60K miles (five years ago) and noticed the blades were pretty worn. Did the blades wear down again? Is there a way to check the blades without emptying the cooling system? What's wrong? Thanks.