hi david
the settings under 'document setup' → 'display' and 'preferences' → 'display' don't change your final document. this settings are here to define how the working file shows on screen.
the page color is meant to simulate the color of the paper (or other material the document will be printed on…)
for your document the black background has to be printed. you have to define it in this way, not as a look of the printing paper!
of course you could print it on black paper, but not in a cmyk color space. in this case you need 'white' as a spot color – and for working on the file you tell scribus the paper color under 'document setup' → 'display' – this is wysiwyg!
utnik