If you’re looking to have a community within your Blog, you should really be using BuddyPress. It adds a social/community layer onto your existing WordPress MU installation.
Like Chrishajer said, bbPress is not for that – it is simply forum software.
I’m using bbPress for a community – but it’s a Discussion-based community. So a forum is great for that.
Visit my forum to see what I mean: http://www.inniosoft.co.cc/devstation