[Ipstenu (Mika Epstein)]

I’m assuming you’re talking about something like this:

Joe and Jane have blogs on your MU site.

Joe posts ‘My life’ in Joe’s Blog.

Jane posts ‘My dog’ in Jane’s Blog.

Both posts appear in the forum ‘Blogs’

The problem I see with that is, eventually, Joe and Jane may post the same title and goof things up. Regardless, you’d have to make either a plugin to post from WP for each blog post, or to suck posts in from BB. Neither method would be perfect I suspect.