I didn't get it right away, but now that I do, it's pretty ingenious. Let me see if I got it right. Build a big ball in space. If the ball was empty, starlight and cosmic background would heat it up, the inner surface would emit photons, and they would bounce around the shell - so you're back to square one. But the black hole at the center can absorb those photons without becoming hot. And the photons are unusable because they are ambient.

On the other hand, there is now a temperature difference between the inside and the outside. Can it be used to make usable energy?