Black smoke is too much fuel/too little air in combustion. So you start with as the other man said, air filter, also make sure you have no restriction in intake before filter. Go on to tappets.
Depending on your transit, you may have a clogged intake (banana shaped going across rocker cover), turbo or EGR problems. Then you are into fuel injection problems, these really needs a garage. You might start with timing or injectors and end up with the pump itself
Kevin Handley, June 2007