painting aluminum does not make it a better radiator. Possibly anodizing, and some audiophiles have argued with me as such, but simply painting with a flat paint does as much to insulate as it does to promote radiative dissipation. Also paint + heating/cooling + saltwater = flaking paint. I have pictures to prove it. Also any minimal amount of convection completely eclipses radiative cooling.
Also I had the pleasure to spend 2 days at the Cree factory in Racine, WI (commercial lighting, but I learned a LOT about their chip manufacturing process) and I pointedly asked them if they considered using a black anodized heat sink instead of clear anodizing and the answer was an emphatic "no". Whether indoor or outdoor, convective cooling and clear anodizing is all they needed. They also pointed out that certain white finishes are actually better radiators than some black finishes.