I thought my serpentine was slipping but now I don't know. I was driving between 65 & 75 mph when I noticed the exceleration was hesitating big time going in and out I was on a bridge so I just kept going. On decline I let off the gas and the vehicle died out. It took a minute to restart now I notice black moisture coming out the tailpipe. Im not sure what's going on

What year and engine?

Check for codes and pending codes. May be related to a failed DPFE Sensor, MAF Sensor, Transmission Range Sensor, or other.

Acceleration problem,, fuel, timing,electrical,, have to know more about the vehicle...