If you look in /etc/X11/Sessions of /etc/X11/gdm/Sessions you will probably find the scripts used to launch the desktops/WM's you have installed from GDM. You can create a script here called "blackbox" and simply enter the same contents as your .xinitrc, or something like this:
# GDM Session script for locally built blackbox
exec /etc/X11/Xsession /usr/local/bin/blackbox
Make it executable (chmod a+x scriptname
) and restart GDM and choose blackbox from the menu. The contents of your session script can vary quite a lot, sometimes a simple "exec blah" will do.