Hello Kirk,
uninstalled completely Photo Mechanic 4.6.8 beta 1 with attachments (plist, cache...) as suggested.
Reinstalled and working without problem (ingest, view and select Raw, edit with photoshop, add metadata, send FTP...), if I not play the slideshow. All OK. Quit and reopen PM without problem.
If I play the slideshow all goes well, but when I quit Photo Mechanic and reopen it, reappears the black background.
Thanks,
jose.