Actually, my .xinitrc just says
but I have everything else, like TWM, commented out. That gets me blackbox when I start X - and a nice thing to do is load up any other apps you want in blackbox, like xterm, by putting it like
xterm -various args
other apps -various args
But as far as getting it on the gdm menu, if it's anything like the kdm menu, I had to edit /opt/kde/share/config/kdmrc, and added blackbox to the 'session types=' section so that it would show in the drop down menu in the graphical login - and I put it first, so that it would be first in the list. gdm's probably something similar.
Maybe that's not the way you're supposed
to do it, but it all works, anyway.