The HOMEPAGE (http://blackboxwm.sourceforge.net/) for x11-wm/blackbox is unavailable (404 or otherwise unreachable).
https://github.com/bbidulock/blackboxwm maybe?
Was fixed a while ago as https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=722df651762e5a8852fe924c33d707dde02532df