Slackware comes with Blackbox. Heck, Slackware includes TWM (Tom's Window Manager
, one of the earliest window managers, commonly referred to as Tabbed Window Manager).
Exit the GUI to the command line and enter xwmconfig. Use the arrow keys to select BlackBox and press OK.
Or, from the Fluxbox menu (assuming you haven't modified it), select "Fluxbox menu-->Window Managers-->blackbox" and Fluxbox will throw you into Blackbox.
It will give you a reason to appreciate Fluxbox.