As we know, from a surprising corollary to Kirchhoff's law of thermal radiation, just as darker* objects absorb more light (and therefore energy), darker objects also radiate more light (at lower temperatures, in the infrared) and therefore energy.
Observe the following photograph of the ISS:
International Space Station photographed by an STS-134 crew member of the Space Shuttle Endeavour on May 29, 2011. Image Credit: NASA
I count ten radiators, all of them light-colored or metallic. The same sort of material appears on the Space Shuttle's radiators, which are on the inside of the payload doors:
The Space Shuttle Endeavour approaching the ISS during STS-118 on August 10, 2007. Image Credit: NASA
What this says to me is that these radiators are inefficient.
From research for this question, I found that the consensus of the internet for terrestrial radiators (e.g. for heating) is that a dark coating isn't worth it: just use a larger radiator and, in the presence of an atmosphere, most heat transfer is by convection anyway.
However, in space, efficiency is paramount. Someone needs to check up on all those panels, fix them up, repair damage when possible, &c.--to say nothing of lugging all those precious grams into orbit in the first place. If you could, say, put a carbon nanotube layer on them and use one fewer, wouldn't you?
So my question is this: why, with the particular examples of the ISS and Space Shuttle, aren't radiators jet black?
* Note: darker here is not necessarily in the visible range. If these panels have low albedo in the nonvisible range, for example, then that might be better than some blacker material with a higher albedo in the nonvisible range. This would certainly explain these radiators.