Home made fan shroud.

If painting radiators black helps, how come every air conditioner condenser on a house or business is unpainted. Color only matters when there is light exposure. The reason all the outside a/c condenser are not black is that they would absorb heat from the sun, thereby lowering their efficiency.

Assuming that the radiator material, such as copper or aluminum is a better thermal conductor than paint, then paint can only lower the efficiency, not increase it. It will act as an insulator. Color is irrelevant. The only way for paint to increase efficiency would be for it to add more surface area.

In practice, the radiator paint is so thin there is probably not much of an effect. The only reason radiators are painted is for aesthetics. Brass tarnishes and sloppy soldering looks bad.