What kind of oil do you use? In reality, it's not a bad thing if it turns blackish. That means the detergents in the oil are doing their job keeping the motor clean. If it is freshly rebuilt, you don't have to worry about breaking any sludge loose and creating problems. Do you run a PCV valve? If you don't, if it is clogged, or the car is running very rich, it may be getting contaminated prematurely.