I had this issue and tried all of the suggestions here, and on many other threads. Reset SMC, PRAM, tried different computer, cable, brightness settings, etc. I also tried unplugging it and leaving it for a few minutes. Nothing I did would wake the display up. The display was detected: I could hear music through the speakers; I could see the preferences and adjust them; the only issue was that it was not showing the video signal. Last night I had given up and started looking for a new display, a Dell or something. So I unplugged my Thunderbolt display and went to bed.
This morning, I plugged it back in and tried again - it came to life immediately! Inexplicably.
So, I have arrived at the conclusion that the inherent problem with these displays is related to temperature. It's hot here at the moment, and my office is not air conditioned. The back of the display is also pointing out of the window at the morning sun. These displays have fans inside, so need to be running as cool as possible; I think they need to be turned off completely now and again, and cooled down sufficiently.
Hope this helps someone. Please comment if this also fixed your 'dark screen' problem.