This one is really strange. Two days ago the wallpaper on Ubuntu 13.10 simply went black. I have repeatedly tried to change it in the Appearance menu back to an image, or one of the default wallpapers, but it remains persistently black. I have no clue of what to do. Thanks.
Update I: some things I have tried that did not work:
gsettings set org.gnome.settings-daemon.plugins.background active true
Purging the gnome3 PPA
Setting the wallpaper with Firefox
I have desktop icons, thus I don't think that could be a cause.
Update II: I captured a video of this strange desktop behaviour.
Update III: other fixes that are reported to work on Ubuntu 13.04, but are unsuccessful on Ubuntu 13.10:
Unticking the "show desktop icons" option (it doesn't exist in Ubuntu 13.10)
Unticking "Have file manager handle the desktop" in Gnome Tweak Tool
Changing the GTK theme to Adwaita
Update IV: Logging in to a Gnome Flashback session I get the same behaviour with the wallpaper.