The accumulation of soot in many types of diesel engines is quite the norm, but there is such a thing as too much soot. Unfortunately, visual inspection is not an adequate method to determine how much soot is present. The most common method for soot determination is infrared spectroscopy. Most oil laboratories provide this test at a reasonable cost. It looks for an increase in the baseline attentuation of the IR absorbance of the sample, and correlates that to a soot level. Most engine designers can specify a maximum soot level (typically in percent wt/wt) and you can use this as a guideline to change your oil out. If you have a lot of engines, and would frequently need to check soot levels (such as fleet applications), instruments like the Wilks InfraCal soot meter are specific for just measuring soot. But if you are just trying to troubleshoot or occasionally check if you "black" oil is normal or has too much soot, I'd suggest a simple lab FTIR test.
Hope this helps.
Rich WurzbachMRG Power Labs