Modern engines bear little resemblance to engines of even 20 years ago. Oils today bear little resemblance to oils of even 20 years ago. DO NOT UNDER ANY CIRCUMSTANCES EXPECT TECHNIQUES THAT WORKED *THEN* TO BE HELPFUL TODAY!!!
Diesel oils are required to be able to hold soot in suspension so that it does not form deposits in places you don't want deposits to form. The soot particles are so tiny that they have no effect on the ability of the oil to lubricate things as it should, indeed too small to be filtered out. But man, is that soot black. It is so black that only tiny amounts of it will turn the oil visibly black in a heartbeat. SO DON'T BE CONCERNED THAT YOUR OIL IS BLACK. In fact, if it wasn't black, THEN you should be concerned because the soot is accumulating somewhere and you are soon to need expensive repairs.