Hmm, this is an interesting idea. I agree it is a little different than what you’d experience on other blogs. The blog is handled by WordPress and the discuss side by Discourse. To get the two to work together we use the excellent WP-Discourse plugin.
Since the comment field is handled by Discourse we’d have to figure out some way of using the API to generate a field where you could enter a reply. That might also require showing all comments on the blog side so you could get the context of the discussion. Hmm.
Looking around a bit it sounds like the developers of Discourse are against this idea as a design principle.
One thing we could do is show more comments on the blog to establish the conversation up front. Perhaps even some custom development work, but I don’t think that’s a priority at the moment.