If it's black that usually indicates the fuel/air mix is not at the correct ratio. This is most commonly caused by a remap which has altered the ECUs pre-defined factory settings, but not maintained the correct fuel to air mix.
The only way to know for sure would be to take it to a reputable remapping company, get the to read the current map, see what improvements could be made, then see if any smoke it present with the new map.
It could potentially be other things which affect the fuel & air entering the engine: MAF, air filter, fuel injectors, fuel pump, etc, however these are less likely (IMO) than a remap issue.
Personally, and this is just my 2p, I would be cautious about buying a car given that:
- The owner doesn't know if it's had a remap or not
- The owner drives the car 'hard'
Not to doubt the integrity of your friendship, however the owner could know full well the car has had a remap and doesn't want to admit to it. If he really doesn't know if it's been remapped or not I would question why he's never sought to find this out if it's creating black smoke. The latter is not usually something potential buyers are privy to before a purchase, however if a car has been driven hard and not maintained regularly and thoroughly there is an increased risk of wear on all front, apart from the rear seats...
