Here's some followup if anyone is interested. I think I may have serendipitously stumbled on to a solution for this quandary.
I recently had to use a data recovery tool because I inadvertently deleted a folder from the trash. To use the tool, I had to "prepare my system for data recovery." I have a late 2013 iMac, 21.5 inches, High Sierra 10.13.3. Apparently this preparation is required for the tool to work properly for High Sierra. Incidentally, it did not, as I could not recover the deleted folder, but that's another issue.
Since I followed the instructions for this preparation, though, the issue of black screen when logging out of Guest User is no more. When one logs out of the Guest User account, the normal login window appears. It may be nothing more than coincidence, but if anyone wants to give this a try... please have at it and let me know if it works for you.
These were the instructions for preparing the system for data recovery.
1) Restart the computer.
2) Once you hear the chime, immediately press and hold down the Command (flag on windows keyboard) and R keys at the same time.
3) Keep holding down the Command (flag) and R keys until the computer enters recovery mode. Then release the keys.
2) Click on Utilities from the drop-down box and select "Terminal."
3) Type in the command "csrutil disable" and then hit return. Restart the computer.
While I never could recover the deleted folder, since I followed these instructions the problem with a black screen upon logging out of Guest User has resolved. It may be nothing more than coincidence. Or it could be bona fide felicity. Good luck!
