WordPress.org

Does http:BL WordPress Plugin work with WordPress MU?

Yes, it does. If you don't want to give your bloggers an access to the plugin's configuration page, you have to modify the source code slightly. Find a line containing plugins.php and change it to wpmu-admin.php. It should work fine. If it does not, do not hesitate to inform me about it or provide your own patch.

Is there an easy way to contact the developer of this plugin?

Of course there is. Visit author's website in order to find his e-mail address and Jabber ID.