As sad as this is to say, it will be alot easier for you to simply install the BB RPM at this point, distros do this MUCH different than others, so you might just want to go that route. However, I wouldn't
I'd forge on, learn the "standard" way and then at least you know.
create the file in your users home directory (if your username is sam, most likely the home directory will be /home/sam so the file will be /home/sam/.xinitrc) if one doesn't already exist. If one does exist, please post the contents of it here. I really think you should post it here before making changes, but here's what I will probably be saying next:
Comment out (meaning put a # sign in front of) the entries in there referring to KDE, then create this entry:
exec blackbox (if you want to enter full path use: "whereis blackbox" to locate it)
Then logout, and back in, startx as this user, and it *should* work.