Not claiming to be a diesel or gasoline guru but through my 65 years of seeing engine oils I am of the opinion diesel oil will turn black quicker than gasoline. I've not done a study on this so shall not present any documents verifying my beliefs. I am aware engine condition is a large factor in how quick this happens. My year old Toyota hardly has any color differences between oil changes. My fresh ringed (no oil use) Yanmar oil will be starting to turn black after 10 or so hours. In my experience I would say it "unusual" to have a diesel engine run over 50 hours and not turn black. This is just my opinion and I'm sticking to it.