Hmm it's a bit of a toss up, it will hurt convective heat flow (extra material with low thermal conductivity and extra interfaces) but the radiation, yes black is good at absorbing and dissipating by radiation. In standard fan/rad setups convective heat transfer is without a doubt way more important than "radiation." But in this particular design a radiative heat transfer
may be more important although I doubt it. The only way radiation could be primary to convection is if there is no fluid flow around the radiator, but the heat will cause the air to rise and hence you have fluid flow around the radiator, meaning convective heat transfer is still probably the primary heat transfer occuring.
So, will painting it black help or hurt? Depends on a couple things, mainly temperature difference - and other radiative sources of heat. If you have a grow-op with some big grow lamps near your computer, the black surface will, absorb that radiative heat more quickly. Since radiative heat is a quartic function of temperature the difference in radiative source temperature and the object absorbing the temperature makes a huge difference. I doubt quite highly dissipating heat by radiation is going to make a lot of difference.
But will the painting hurt conduction...err convection - yes. A lot? No. But the thickness of the paint does make a huge difference. The temperature gradient in a thin poorly conducting material can be as steep as one in a thick highly conductive material.
I think the overall consensus is that paint doesn't hurt temps enough to justify ugly rads - personally mine's not painted, I'm ugly
. This passive radiator does increase the % at which the heat is radiated but when we're talking < 50deg C the heat radiated isn't all that much (in comparison to 1000deg) Basically the hotter you get the more radiation takes over as the primary method of heat transfer - just due to the physics.
Hopefully this adds some light to your discussion.