Black smoke coming from exhaust and car cutting out?

i just bought a 1998 vauxall astra which was cutting out at low revs a mechanic said it was the breather pipe and head gasket which he changed and skimmed the head gasket.i drove the car on a motorway today and it was fine then i parked up and got in a yellow engine sign with a lightning bolt came on the dashboard and the car cut out again but started again first turn of the key but again cuts out on low revs.now its blowing black smoke only when i put my foot down on the accelerator and ten it goes does anybody know what the problem is because i paid �150 for the head gasket to be skimmed thanks