The basic function of the engine oil is to lubricate as well as to clean the engine components. Engine oil cleans the black carbon deposits; soot etc. formed on the engine parts due the combustion of the fuel (diesel/petrol) and thereby becomes black. It does not mean that the condition of the oil is not good. Oil can still be used. However ensure that the oil is changed at the drain interval suggested by the car manufacturer/lubricant supplier.