Ok. I take back the sleep button workaround to the black screen. Just had this happen to me again. Apparently it's only when I disconnect the keyboard and the system goes to Continuum where this bug comes about. Seriously a pain. Coupled with the black picture password, I'm almost to the point of going back to 8.1.
I just experienced the black screen. Literally unplugged powder cable, pressed piewer button, then pressed again. Normally following this, I would hold thenpoewer button, and the slide to shut down action would appear, and I could properly shut down. This time, it flashed a couple of times and then eventually showed the lock screen and I could log in as normal. Either way, its a pretty damn annoying bug. Justblike that pointless alt Ctrl delete sequence just to access the password login screen.