There are a few possibilities. My first instinct is that the thermostat is stuck open. However, before replacing it go through a few steps to eliminate other possibilities to ensure you don't waste your time or money.
Has your anti-freeze been serviced recently? I know Chevy uses the Pink long term anti-freeze, but it is something to eliminate because air can get trapped in the system when the anti-freeze gets changed or gets replaced or topped of as part of another service. If you had any repairs done recently, that may be the case. Sounds like you have checked it, but just want to make sure. As you near the service life of the GM anti-freeze, make sure that you replace it because it deteriorates very quickly after its recommended change interval. I think it is 100,000 miles but time is the really critical factor.
As it gets older it tends to cause issues such as corrosion on cooling system components that will cause issues like a thermostat that is stuck open.
So, if the blower motor works and the mode control works, e.G. Changing from feet to face works and blows through the correct vents, replacing the thermostat should not cost much and is not hard to do.
You can find links on this site to help you through the repair or to help you find a reputable repair shop near you.
Good luck and let me know how it goes.