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.

Whenever you leave behind failure that means you're doing better if you think everything you've done has been great you're probably dumb
-Louis CK-

'53 GMC