1997 Other Opel Models rough idle, black smoke

Engine Performance problem
1997 Other Opel Models 4 cyl Two Wheel Drive Automatic

Black smoke occurs after i took my Opel Vectra B to a car service shop to fix rough idle problem. No idea what they have done and instead they claimed my car was having a black smoke since the first day i endorsed the car :( It seems that my car has more problem

Could it be caused by fuel regulator problem ?
How to test and fix fuel regulator ?

Any advise is really appreciated,

Thanks