Parallels Support and the Development Team continue to work on the black screen, seemingly "blocked", virtual machine issue. Please note that the virtual machine remains active even though there is a black screen. As for shutting down the VM, only a hard Stop will do it. I am a few steps from having the teams tell me to do a clean install of MacOS High Sierra...
For individuals that continue to have trouble updating to the current version of Windows 10, the following suggestions may help. The black screen issue may continue to persist, but at least the update should take place.
The disabling of Fast Startup is a workaround that works in the majority of cases. As
Rollinger points out, however, a black screen may appear when executing a reboot during installation of the Windows 10 1803 update. This may or may not be related to the same issue of a VM that has successfully been updated. My suspicion is that it is similar, if not the same.
I keep backups of all my VMs. When things don't go as planned with updates, I start over with the backup copy of the VM and start making changes to the update process.
Using the
Windows Setting > Update & Security > Windows Update > Check for updates process is tedious. The alternatives of creating an ISO and booting from it prior to an installation attempt takes about an hour or two, sometimes the rest of the day! For recalcitrant VMs that either refuse to update or perhaps not even show the latest update through the tedious processes listed above, an alternative is to go directly to the
Microsoft Catalog Update site and select the update you want. This is much faster, especially when you have more than 2 or 3 VMs to update. Also, you can use the same update file on all the VMs (or bare metal computers).
The links for getting updates from the Microsoft Catalog will follow. The first link starts with the Windows 10 and Windows Server update history, followed by specific Updates for Windows 10 version 1803, and finally the Catalog:
https://support.microsoft.com/en-us/help/4043454/windows-10-windows-server-update-history
https://support.microsoft.com/en-us/help/4099479
http://www.catalog.update.microsoft.com/Search.aspx?q=KB4103721