Not to mention, ive also heard that a black radiator disapates the heat much better over an unpainted aluminum one. Why? I dont know, buts that seems the be the greater opinion on other varios sites.
Engineer's hat on here. It's a thing called "black body radiation" and reflective surfaces such as aluminum do radiate less efficiently than black surfaces. Although in the application of a radiator (probably more correctly in scientific terms called a convector) the forced convection (air flowing over a hot surface and scouring the heat away) is WAY more effective than the radiation component of heat transfer.
For practical purposes, you wouldn't see a significant difference between an unpainted aluminum radiator and an identical one that's painted black.
