So I just upgraded my Dell XPS 13 laptop from Ubuntu 13.04 to 13.10, and immeditely the first thing I noticed that all of my desktops had a black background. and manually changing the background wallpaper took no effect. Turns out this is a common problem. In my case it turned out to be related to Gnome, which I found it to be rather interesting giving that a Gnome specific setting will case this in Unity.
Fix:
gsettings set org.gnome.settings-daemon.plugins.background active true
Reference: http://askubuntu.com/questions/287571/desktop-shows-a-white-or-black-bac...