I don't think it's possible without changes to Wikia's back-end architecture.
Currently each reply to a blog post (and each reply to reply) is a completely new article in the Thread space.
Basically Wikia tried to re-use as much of already-implemented architecture that they could. Given that, I kind of understand why replies to blogs are threads, but it still seems kind of stupid that replies to replies are also threads.
It would make more sense to me if blogs were essentially Message Walls with a header (the blog post). Then you could follow (via watchlist) it by watching the "wall." But... it's not, so something would have to change.
Also, as Saftzie said, this is something not easily possible for a Dev script. Even if it existed, it would have to possibly run for hours and clog your internet connection, and it wouldn't work for old comments unless they were watchlisted.