My personal opinion is that radiators were originally painted black because they were made from copper, and copper goes ugly very fast when exposed to moisture and air. It was probably also for looks, as you get that "empty" look when you can't see the radiator/intercooler. Lately I can't remember when last I saw any new car with a raiator/intercooler that's been painted black.

Think about aluminium foil that you use when cooking, it's got a shiny side, and a matt finiish side, depending on what you are cooking, mostly you will use the matt side towards the food, so that the direct heat from the heat source is reflected away by the shiny side, and you don't get food that's burnt on the outside, but raw on the inside. So if even that slight difference in colour can make a difference, just think what black would do, it would attract more heat even just standing in the sun.

Keep it shiny, looks better anyway...