blackbox/fluxbox sit directly atop of Xlib, porting it to a toolkit would be difficult.. one project "bb4win" clearly realized this, they rewrote almost everything from scratch.
Now, is using Xlib a bad thing? not really.. every modern OS has a native X server available, even if done by a 3rd party, so technically.. it's quite portable..
Still.. some people wouldn't like that requirement.. one option would be GDK, the low-level API used by GTK+.. or GTK+ for that matter.
Both GTK+/GDK are under the LGPL vs the GPL, the "lesser" of 2 evils if you will.. but I've given it too much credit.