I think that the only reason they paint radiators black is just to allow
the chrome plated grille at the front of the car to stand out more.
That is, it's strictly a bling thing.
There are three ways that heat moves; conduction, convection and
BUT, radiant heat transfer is tiny at low temperatures, and only becomes
important when we're talking about temperatures of several hundred
degrees or more. Below that, conduction and convection are really the
only games in town. So, I may be wrong, but painting the radiators
black to increase radiant heat loss doesn't make any sense when at the
relatively low temperatures of a car's cooling system (212 to 250 deg.
F), it would be far more effective to just use a little bigger radiator,
and lose way more heat to the air by convection.
Besides, if you think about it, the cylinder head on a motorcyle engine
gets way hotter than any car's radiator does, and yet MOST motor cycle
engines are the natural colour of the steel or aluminum the engine is
cast from. Radiant heat transfer is greater at higher temperatures, so
if it's not important enough to anodize the aluminum a dark colour or
paint the steel black on a motorcycle engine at 500 deg. F, it sure
won't be important to paint a radiator black at 250 deg. F.
So, I don't know why they paint radiators black, but to do it to
increase radiant heat loss on cars but not on motorcycles just don't
make no sense no how. So, I doubt that radiant heat loss explains the
black paint on my car's radiator.