You want as much heat from the core to be transmitted to the air as possible. Black surfaces dissipate heat much better (and absorb it too - it's a two-way process) than bare aluminium or copper surfaces. In fact, a perfectly black surface will get rid off 14 times as much heat as an unpolished aluminium surface, and 1.5 times as much heat as an oxidised copper surface. With radiators made from these two materials, it makes sense to make sure that the radiator stays as black as possible. A can of matt black spray paint will work wonders.