I had a similar problem - my external display would go blank and refuse to wake back up (with my Macbook in clamshell mode).I've noticed that my external LCD randomly went black a couple of times. It would come back after a couple of seconds. It seems to happen while playing a game for a while (WoW mostly). Anyone have any ideas on what it could be?
I have been experiencing similar issues on my 15" Retina MBP. Having spoken with Apple they recommended turning off completely and holding Cmd+Alt+P+R and turning the power on. once you press the power button, keep holding the four buttons and you should hear the start up tone, then you will hear the tone again. The guy on the phone asked me to hold it still and I heard the tone a third time before he told me to release.
My understanding is that this clears the PRAM. It probably didn't need to do it twice, but better safe than sorry.
Anyway, probably not related to the original issue, but I found this thread when searching, so hope it helps someone else. No ill effects after 30mins or so, so I think it is fixed.