You don't know how much we've searched for a forum system that integrates seamlessly with Wordpress comments (i.e., blogposts are threads and blog comments are posts). Unfortunately, there is none.
What we can do is close comments entirely on the blog and have a link at the bottom of every post manually directing to a comments thread here. It's not elegant: it won't display posts here as comments there, and it might kill guest participation (unless guest posting were enabled).
Is that a bigger harm than bifurcated discussion though?