I battled the 30-second black screen constantly after both first and second iOS 11 updates. Hoped the second one would fix it..but no. After updating iTunes, deleting dev apps not compatible with iOS 11, and rebooting dozens of times, I finally gave up trying on my own. I backed up my iPhone 6s Plus to iTunes, and took my phone and MacBook to Apple, (Simply Mac actually), and they did a “reset” and “restore from iTunes backup “.
Fingers crossed..that was just yesterday..but so far no more freezing and black screen.
I am still having issues with my ‘received’ text message bubbles disappearing under the response line, under/behind the keyboard so to speak ..when I type my response, and I can’t pull them back up into the screen. I have to exit messages and go back in. Not as frustrating as the 30-second black screen was however. Maybe NEXT update