Why is my desktop background black after upgrade to 13.04?
The following line pasted into a terminal did recover the background image for me:
gsettings set org.gnome.settings-daemon.plugins.background active true
Hope that helps you too...
I recommend to anyone still with this black desktop problem unsolved, to try logging in to another account and checking both Unity and GLX plugins are activated. IE 'opengl' & 'unityshell'.
Use the following command with the get option, instead of reset, to list them:
gsettings reset org.compiz.core:/org/compiz/profiles/unity/plugins/core/ active-plugins
But no guarantees!