Fluxbox Tips and Tricks Transparency
You can change the level of transparency (actually it is 'fake' transparency) of the menu or the toolbar by right clicking to bring up the fluxbox menu then
You can click on the value there to adust it. "0" is fully transparent, "255" is solid. Values in between have varying levels of transparency.
If you find that the menu's transparency does not take effect immediately, try restarting fluxbox.