I'm interested in switching to something like fluxbox or blackbox hoping I can pick up some performance, but what I don't get is how exactly how thoes window managers in particular work.

For instance they both say they offer "GNOME support", but wouldn't the idea of switching to something like Blackbox be so that I can get rid of GNOME, or does it need it to work properly?

AFAIK, black/fluxbox are just window managers which manage the look and the ease of use relating to ONLY windows + a little add-ons, while GNOME is a huge desktop environment which includes many things including a default window manager(i think it is called sawfish or nautilus???).... so you can switch the window manager component(?) of GNOME to other window managers....

On the other hand, if you use black/fluxbox only, you will have the minimum functions to handle your windows and menu.... excluding the icons, GNOME foot mark, etc....

Sorry I couldnt explain it well but you may try it out urself to have better understanding

The "standard" window-manager for Gnome is "sawfish"

I've used fluxbox too - it is a very good and fast window-manager - and fairly easy to configure.
But you will have to configure it, if you want to use it as a working environment - meaning:
you will have to add (almost) every programm you want to access - using just your mouse - yourself to the menus. Once this is done - and this can be a lot of work - it's really easy to use.
But if you still want to use apps from Gnome or kde, there is not much point in using this WM, because what makes gnome and kde a heavy load on the system is the applications themselves - and not the window-manager, that is used to --> manage these (applications) windows.
Gnome or KDE-Support in fluxbox ... does just mean that you can use that WM for managing Gnome/KDE-Applications without loosing the functionality provided by the "standard"-WM's they are designed for.