Mac on High Sierra won't shutdown cleanly: black screen with mouse
Restarting in safe mode allowed proper shutdown:


  Start your Mac, then immediately press and hold the Shift
  key. The Apple logo appears on your display.
  
  Release the Shift key when you see the
  login window.


I was able to use safe mode used this to update to 10.13.6 (problem still existed) and 10.14 (problem resolved -- can now shutdown).