*thinking*
Such a plugin would need to know the database data like name, login, password, table prefixes etc., it would need to know which forum user it should use for posts.
When a new blog post is made, the plugin would need to create a new thread in a certain subforum of the board. It would be also nice if the plugin could keep blog post and forum post synchronized, so that a modification in the blog also shows up in the forum.
It would need to provide "Comment in forum (# comments)" link to the connected forum post, and to show the number of "comments" it would also need to count the replies in the thread.
*thinking complete*
Hmm, maybe that is a little much effort - but it would be a cool plugin nonetheless.