You're right, I did only count the front of the radiator and not the whole thing. That might get you a little more than double: the back surface plus the tanks and sides. And yes, black is the best for radiant cooling. But I wouldn't expect the surface area of the fins to make any difference, since a material is just as efficient in emitting as it is absorbing, and when one fin emits some radiant energy, the next fin over that it's facing is going to just reabsorb it.
I was just trying to see for myself how much of an effect the radiant cooling has. Since the airflow is hundreds of times more efficient at removing heat, I don't think it makes any practical difference.
