blackhole: Fix wallpaper.color.primary to use defined black

[haaspors]

This fixes a problem when using blackhole look and regolith sway, where the /usr/bin/regolith-set-swaybg script will execute

wallpaper=$(trawlcat regolith.wallpaper.file "")
color=$(trawlcat regolith.wallpaper.color.primary)
swaymsg output "*" background $(xmllint $wallpaper --xpath "string(*//size/text())") $mode $color

which again ends up in

Error: Invalid output subcommand: black.

from sway-output man page

...
color should be specified as #RRGGBB. Alpha is not supported.
...

[kgilmer]

LGTM! (@ploum FYI)