One for all you derv lovers.

[Kezzieboy]

I think their point is that when you have airflow, you're fine anyway, as Dumdum pointed out, the coldside stays very cold when you're travelling at speed because of the cooling capacity of the huge volume of air passing through the fins. So painting it black stops it from getting chronic heatsoak if you stop/slow right down, without effecting efficiency while moving.

It's not exactly peer reviewed science, but it's still a cool example of the emmissive properties of black bodies compared to something reflective.