Black oil in a Diesel engine is totally normal, and nothing to worry about. Even the best tuned Diesel engine will produce a small amount of soot when running, and the tiniest bit of blow by the piston rings, and the oil is instantly black. This is very different from a gasoline engine where soot indicates a cumbustion problem.
Do not let anybody sell you an engine cleaning just because your oil is black. The best thing to do if you are concerned about the condition of your oil is to have it lab tested. It is fast, and not at all expensive. That will tell you if it has normal levels of soot and other contaminants or if something is wrong. The best thing of all is to do such testing routinely so you can see changes with time.
Fort Lauderdale, Fl, USA