Finally resolved my black screen issue!
After replacing the Pi 3B, the SD card, the OS, using the latest MM software, much internet searching and a great deal of help through this forum it appears the problem is being caused by the jpg photos being added to the wallpaper module. Changing the photos from large tif files into jpg files using Adobe lightroom creates a highly compressed jpg file that the wallpaper module cannot deal with properly.
Reducing the resolution of the files during the conversion seems to create a less compressed jpg file that the Wallpaper module can deal with.
Hope this helps others with the same issue.