Similar to issue "mac book pro sierra does not shutdown stays idle with a black screen and mouse".
Apple logo > Shutdown leads to black screen with mouse cursor. Stays like that for hours.
Realized this problem because it wouldn't install Mojave (also won't install 10.13.6 update). Seems I'm stuck on 10.13.3.
I tried verbose mode, but get the same black screen with no text on shutdown (did see lots of black screen with text on startup).
I get a bit more information when shutting down from Terminal:
$ sudo shutdown -r now ... shutdown: / is busy updating; waiting for lock
Stays like that for hours. I used Activity Monitor to kill some random applications (starting with launchd) and it proceeds to shutdown. I narrowed it down to killing kextd to complete shutdown.