Radiation- Internal/External?

When I worked in the aerospace/satellite industry, we painted all our various electronic enclosures inside the space craft with flat black paint.

Out there, we didn't have any convective heat transfer, no air.

My guess is that this would hold true in Earth's atmosphere, but the air here would also distribute the heat more evenly.

I don't do thermodynamics.

Maybe Vader knows. He wears black, you know. Of course with him, it could be a style thing.

Despite its name, a radiator does rather less radiating than it does convecting. So the strongest quality of the paint has to be that it is highly thermally conductive. Its colour is of little relevance.

To improve the performance of a wall-mounted radiator, the easiest way to do it is to place a retroreflective surface on the wall behind it, preferably with a thin layer of insulation between that surface and the wall.

Commercially-available materials may be had at any major home improvement outlet, and are relatively inexpensive.