I found a workaround.   Even though I had previously disabled Fast Boot, it appears that Windows was not shutting down completely upon invoking shutdown, and that this seemingly caused random problems upon powering back up, and thus the black screen. I created a Shutdown Shortcut with the target: %windir%/System32/shutdown.exe /s /t 10  When using that Shutdown Shortcut, the black screen problem was completely fixed.   I suspect that, for many users who HAVE disabled Fast Boot, what I described is causing their black screen.
After doing a clean Windows 1809 install (OS Build 17763.316), I am now getting a black screen after some logins, which can only be solved by doing a hard shutdown using the power button.  BIOS is the latest: 1.9.0.  Boot mode is secure UEFI.  I don’t use Dell’s application delivery software because of its intrusiveness.  I’ve read through these forums and have tried several solutions without success:   Some observations   Does anyone know of a solution that they think will work?   Thank you
XPS 15 9550 Black Screen After Login