Painting aluminum black does not help it to disperse heat, by coating the aluminum you are holding in heat, the main reason a copper radiator in painted is that they want to hide their ugly work and no one wants to drive around with a car with a green radiator,if you put on a black jacket in the shade when you're hot do you feel any cooler than if you put on a white one, or do you just feel cooler leaving the jacket off, I would not paint an aluminum radiator, I think you would have problems. Aluminum disperses heat very quickly coating it with anything is not going to speed up heat transfer.