Ac issue/black smoke

Hey guys, I have a 2000 Impala. Just had the blower motor and resistor replaced. All worked fine till I drove to pittsburg, Now If I turn on my ac, My belt starts to squeal and Im getting smoke through my vents. Ive read and read and cant pinpoint an issue. Any suggestions what might be wrong, Ive even looked into doing a compressor replacement but the garage tells me thats not the issue. Before I take it back to the garage, Id Like to have an idea, before they start replacing parts that arent needed. Tia....

My guess would be slipping belt (caused from overload from compressor) or slipping compressor clutch. I'd take a look at the compressor. When you turn on the A/C, the clutch on the compressor engages by way of an electrical signal. That would be the only difference in running with the A/C on and not. Could be a weak belt tensioner but usually the alternator will uncover this for you, not the compressor kicking on.