1996 Chevy Caprice o 2 sensor

The guys that rebluid my transmission told me afther a day or so that they broke one of my o 2 sensors but did not replace it and left it in there. So I been driving with that problem with out knowing. It was missing power and idlering crazy and black smoke out the pipes. So I toke it back and they replace it but the problem came back so me thinking that one effected the other 3 sensors so I replace them all and also spark plugs, sparks plugs wires, cap and rotor but still the same problem what could be the problem.