if it's black then that is unburnt fuel, so the easiest thing to check first is the injectors!!
if you take them out of the head and look at the tips you'll be able to see if the tips look o.k. and all look the same. if one is blacker than the others it could be that at fault,failing that you could borrow another set off someone and try them or take them to a diesel specialist and get them tested.
when it is smoking does it sound like the engine is misfiring and is it any better once it gets upto running temperature?