With help offered here, I solved my blank login screen problem. However, I have encountered another issue.
sddm.config man says I should have these three directories:
/usr/lib/sddm/sddm.conf.d (the system config directory)
/etc/sddm.conf.d (local config directory)
/etc/sddm.conf (local config file for compatibility)
I only have the first listed directory so I edited the "default.config’ file there and rebooted.
The login screen now displays the background image and login dialogue box I added to “default.config”. The dialogue box is functioning, except for one glitch: no matter which session I select, the system defaults to a dwm session.
I should add that /usr/share/xsessions shows session info for dwm, jwm, kde/openbox and openbox.
Obviously, I have overlooked something.