No amount of SEO will help this forum back up the google rankings. If they weren't blacklisting it then changing to the new software would have gotten threads back in the usual search terms. They push rehab/Government links to the top of the results now and list things like reddit above this forum. Forums in general have been blacklisted on common search terms. Google will show you social networking and youtube results above everything else. When I'm looking for stuff on forums these days I have to add "forum" in the query.
I noticed another drug forum managed to stay in search results longer than BL but it seems to have been deranked now too. That forum practiced very aggressive SEO and I'm pretty sure google finally caught up and started punishing them for it. The best thing to do if you want to bring new blood into BL is word of mouth. Another good way would be producing content for places like youtube that link back here or promoting it at events IRL.