I find it hard to belive that if I paint my rad black it will help the car run cooler. You are in effect insulating your fins in a coat of paint, therfore restricting the amount of air that can pass between the fins. I'm guessing most are painted for looks primarily. In older cars nobody wanted to look thru a nice big chrome grill and see copper, that turns green when not treated or dull grey as aluminum tarnishes. The black paint if it does offer some cooling properties is insignificant. Just as painting an engine black to reduce heat is. I'm no scienctist but I'll wager a hemi orange engine after a full pass is w/in 5* of a black engine.