My oil does not turn black between oil changes and I go the 100 hours between changes. I use synthetic diesel oil (Mobil 1), so I think the black color might be from the long hydrocarbons breaking in regular oil as opposed to something inherent in diesels. My experience with oil in my car is that synthetic oil doesn't turn dark like regular oil unless you really run it a long time, like 10,000 miles. Whatever it is, black color doesn't necessarily mean the oil is dirty or needs to be changed.