Black smoke would indicate over fueling. I would start by changing the air and fuel filter and see if that cures it. Could be that you have sucked up some crap from the tank so some injector cleaner wouldn't go a miss either.
I had major black smoke problems with my ML270, in the end I replaced the inlet manifold( the flaps must have been worn) and it's now clean as a whistle.
Air filter is straight forward and the fuel filter is easy just make sure you fill it with fresh fuel before fitting or you will have a problem getting it started and use Mercedes filters