AS NIW said - black oil means nothing - actually within reason, black means that the small particles are being trapped IN THE oil as designed.
Oil should be changed each yearly cycle or and "X" hours.
AS Russel said - you need to use an oil FOR diesel engines, now if you used a "gasoline" oil in the diesel engine - would that hurt the engine? NO - but diesel oils have additives in it that capture those small particles that gas oils do not have. Diesel is a very "dirty" combustion - a lot of soot is left behind that the oil must deal with.
10w30 -15w40 ........ brand specific really does not matter.