The theme should be able to define the background color by default from the theme.json file. This is needed specifically when the background of the cover block is an image (and therefore the user can change its opacity revealing said background color). Right now the editor is defaulting to black and can only be overridden by the theme's css

I don't see the Cover block on this table on the docs: https://github.com/WordPress/gutenberg/blob/master/docs/designers-developers/developers/themes/theme-json.md#color-properties

EDIT:

I'm also adding the height of the cover block to this issue for the same reasons. If the end-user can change the values of the height and overlay colors for the cover block, I don't see why we can't have the theme.json be able to set the default values for them too.