1996 Chevy Astro black exhaust upon acceleration

If the vehicle has low mileage, there is the possibility that the truck wasnt driven much and probably never "opened up" on the highway (really accelerated hard). Lack of occasionally doing this will cause huge amounts of carbon to accumulate and when someone finally does "open it up", the results are huge amounts of black smoke coming from the exhaust. This is most common on the vehicles that "little old ladies who only drove to church and back" owned.

The mechanic may be right. These carbon deposits will settle on the oxygen sensors and GM doesnt recommend cleaning of oxygen sensors, only replacing.

Normally after "blowing it out" a few times, the black smoke will subside and eventually stop.