Black smoke is un-burnt fuel as opposed to oil (bluey white colour), does it do it on the overun - ie after lifting off the throttle at cruising speed or does it do it during acceleration? or does it happen whilst maintaining speed and throttle position?
Each of the above would suggest differing problems, but i would start by checking injector pump timing and bleed/return pipes are clear. May not be either from what you have described, but they are the simplest (and cheapest) causes of black smoke in diesels.