I really should know the answer to this, having done a course in heat transfer and basic thermodynamics, but i cant for the life of me derive a satisfactory answer. The question, in a nutshell, is this:
How come the SR-71 blackbird was painted black, yet Concorde was painted white?
MARKETING!
There may have been a relatively insignificant gain to be realized by painting the Concorde black, but passengers would rather fly a bright white bird.
OTOH, the Blackbird was designed to press the absolute edge of the envelope at the time, without marketing concerns...