Diesel engine oil that is black should not be a cause for concern. If you’ve just changed the oil in your diesel engine and notice it is dark black, there is little reason to be concerned. The oil will naturally turn darker as it circulates through the engine and removes impurities.
When you change the oil in your diesel engine, you are draining out only the oil in the oil pan and the filter. There is still residual dirty oil lurking in the engine. Once you turn on the vehicle and circulate the oil, it will mix and quickly blend together to be a darker color than the oil that was just put in.