1) Zellius do that: start you Kde session as usual, then in konsole type "switchdesk" this will pop up a window from where to choose the default WM... but wait you said you compiled it from source? ...uhm not sure then. Being you not confortable with config files and all, I'd suggest to delete blackbox and install the RPM's instead (you can find them at http://rpmfind.net
) for easier management.
2) If you want to try blackbox quickly: echo "exec /path/to/blackbox" > .xinitrc
where /path/to/blackbox is the place you installed blackbox in (usually /usr/bin/blackbox should do). Then startx and you're done.