I am an avid Fluxbox WM user ...
The Fluxbox Window Manager is closely based on Blackbox WM.
Fluxbox WM and Openbox WM - can use the xorg-system-core, with NO desktop environment. Much like what Crunchbang and Archbang have done.
Fluxbox is still closely based on Blackbox. Some say that, Fluxbox is the natural progression of Blackbox. Fluxbox is a basic Blackbox; with a few more things added for user-friendliness. Not to take anything away from the base-Blackbox build ... the BB-Devs did an awesome job. I use many of the BB scripts, themes/styles and apps with Fluxbox -- they seems to be pretty much forward and backward compatible.
Openbox, was also, based on Blackbox but has grown more separate from the BB-base, with using and building it's own configs and apps. Although, Openbox seems to be growing in popularity, with more Linux OS-distro systems using Openbox - as the default Window Manager.
There are advantages, pros and cons, to using either Openbox and/or Fluxbox.
A self-updating 'right-click' Menu
It's own Fb-panel/taskbar (no need for tint2)
A built-in Composer
Does not use a GUI or .xml files but everything is a simple Script
Easy to use and Configure.
There are already hundreds of fb-styles (themes) available.
As light-weight on resources (or lighter) than using Openbox.