I had the same issue when I updated. It took several hours, then everything returned to normal.
I'm not sure if this is something everyone experiences, or just a select few. If it stays a black screen for more than 8 hours, I would really be concerned at that point. I was pretty panicked, then finally decided, "I'll just leave it alone and check on it in the morning. If nothing has happened by then, then I'll panic. I woke up at 2:00am for a run to the bathroom, and lo and behold it was finished and all was well in my world.
You can register for an account, return to this thread, and give us an update on what's happening. Did it ever finish? Did you give up and turn it off? (Bad thing to do, by the way.) Is it still a black screen with something going on in the background? Of course, if it finished and all is well, you'll probably not be back to tell us. (sigh)