ZydoSEO - 9:10 pm on Feb 2, 2013 (gmt 0)
I have seen no indication that a) forum threads have any trouble ranking or that b) a blog post about a topic might actually rank better than a forum thread about that same topic on the same site.
Discussions here, for example, typically have well thought out, substantive responses. There are not that many "one liner" responses here. If you have a forum with active, contributing participants, I see no need to add a blog.
However, other forums like those overrun with spammers (e.g DigitalPoint) might have a much higher percentage of non-substantive threads and response (e.g. "Nice post." or "Thanks for sharing."). Even with a lower quality forum, I'm not sure having a blog with good content would even help as those types of forums often have so many negative signals that a little quality content is not likely to help.
Besides, it just seems a bit weird to me having two different UGC type of formats on the same site. Seems a bit redundant. It's like you'll be trying to develop two different communities on the same site.