That old saw about black colors and radiation only applies to visible wavelengths, and even engines do not get hot enough to glow visibly (the exhaust stacks do). All paints are dark in infrared, main spectrum for heat radiation at normal engine temps, but paint is an insulating substance and that is the last thing you need for getting rid of heat. So generally only lower temperature areas of engines were painted.
Black engine colors may be due to natural patina of steel, or a chemical finish with hot oil. Usually black areas were steel, silvery color aluminum uncoated. Black paint is used on some areas like magnetos, etc. Some engine mfgs did paint crankcase black, but black is not a common color for crankcases. PW and Wright generally used gray, some early aluminum crankcases were left natural.