BML is actively maintained and used, but not actively released and documented. But we're working on changing that.
For now, read the 5-6 year old site: http://www.bradfitz.com/bml/.
But be aware, the syntax has changed. Wait for new docs before proceeding with a project. If you want to download it, it's in CVS here: http://cvs.danga.com/browse.cgi/bml/.
Root is :pserver:anonymous@danga.com:/home/cvspub ... no password. Check out module 'bml'.
Site to be written.