do you have your system set up with init 5 (meaning graphical login)? If that's the case, you should check out this thread here. You will have to modify some stuff like the filenames or the location of blackbox instead of using fluxbox's, but sinc they both are very similar, it should be working fine afterwards:
hope this helps!
note: you might just forget the first 7 step and immediately jump to STEP 8 - don't forget to change everything from "fluxbox" to blackbox
Also when you create the file, you need to know the location of your blackbox - i assume it's /usr/local/bin/blackbox (but i'm not 100% sure because i haven't used blackbox yet)