black or brass

You could paint a radiator any color, blue, green, yellow, black, or leave it brass. It really doesn't matter. The dominant mechanism of heat exchange of a car's radiator and the atmosphere is convection not radiation. It just does not get hot enough to radiate a lot of heat by radiation. That's why fans are used in cars to increase air flow which increases convection. A car's engine will rapidly overheat if the fan belt breaks, right? So whether the radiator is painted black or left in it's natural state of brass, if it functions properly it will cool effectively.