No, black only works for images. That should have been clear by the different wording vs. white and none settings.
That's a compromise to handle the various history of weird and broken ScreenSaver settings.
May I ask *why* you'd prefer a black background for the message?
(My understanding is that the only use-case for this would be "anonymizing" the content; not aesthetics. Hence white was good enough).
EDIT: Took the opportunity to clean up the mess in
https://github.com/koreader/koreader/pull/7496, so, you'll be able to do just that Soon (TM)

.