I just bought a ThunderBolt Dislay (TBD) yesterday. It's attached to a new Mac Mini i7. I encountered a similar issue to yours. But with mine, the display goes blank while the computer is still awake. The sound and everything works, but the display is blank. Moving the mouse and pressing the keyboard does nothing to remediate the blank screen.
It seems like this is a fairly common issue. There's another post on it (link here). It seems like people are sighting bad manufacturing for the problem. Several had taken their TBDs back and gotten new ones.
I hope this info helps.