I'm pretty sure the emissivity of a black body is orders of magnitude lower than what you get from the air passing through the rad. It'll have an effect just so small that it's negligible. The paint would pretty much be a cosmetic or protective addition rather than performance.
On top of that fact is, blackbodies hypothesis is just that, a hypothesis. Blackbodies do not exist in nature, as far as anyone has proven. Not to take anything away from the hypothesis, as it helps explain much about the universe. But the paint on the outside of our 60 year old truck radiators is not blackbodies.
Carl
