Oil smoke is usually a blue-ish color.
Black smoke is actually usually due too too much fuel and not enough air. On a D150, I believe you still have a carb, right? My guess is your choke may be stuck and you are getting too rich of a mixture. You might also check your air filter and make sure it is not clogged.
Correct, Oil=Blue smoke, Fuel=Black smoke, Water/coolant=White smoke
Almost certainly a carb issue.