It is normal for diesel motor oil to turn black quickly. The dark colour is a sign that the oil is performing correctly by keeping byproducts of the combustion process in suspension.
How It Works
The high pressures in the combustion chamber forces soot and other contaminants into the oil, which carry them to the oil filter. The contaminants are what turn the oil black, according to Quaker State.
Higher fuel injection pressures in newer diesel engines produce lower exhaust emissions, but produce more soot, according to "Four Lethal Diesel Engine Oil Contaminants" by Jim Fitch of Noria Corporation This quickly turns motor oil black. In fact, it would be abnormal for the oil to stay clear.
Periodically, the oil must be changed along with the filter. Diesel engines need oil filters just as gasoline engines do. At some point, the oil can no longer keep contaminants in suspension and carry them away from engine parts.