A thin layer wont hurt performance, and guess what, black is a better heat radiator than most colours.
A "black body" is one that is a perfect radiator (and the perfect absorver),at a certain temperature. Actually the nature of the surface also makes a difference. Dull, matt black surfaces are the best radiators.
Shiny surfaces do not radiate so well.
Now apply that to a radiator which is hotter than ambient, then it will counter the new interface layer by being better radiator.
If you wish to say it wont really help that much, i'd say fair enough.
But it wont make it worse.
http://207.242.75.40/derbtech/color2.htm
Check the chart.
Things arent painted black of looks. Well, sometimes they are
.
(
http://www.howstuffworks.com/question32.htm)