well you can add the blackbox into GDM pull down list, to do so go to /etc/X11/gdm/Sessions and create a single file as root
blackbox with contents
Second step is modifying your /etc/X11/xdm/Xsession, open it up sroll down to the case statement
and add there similar entry for blackbox for instance
That's about it, now you'll have blackbox as an entry in your GDM pull down menu upon login.