tl;dr either use a different subdomain or noindex your forums
There are actually two questions here, and the answers should guide your decision.
- Are forum threads 'thin content'?
- Will forum threads rank higher than my blog entry for [a certain keyword]?
If #1 is true, then #2 is unlikely to happen (unless your blog article somehow contains less information than the threads).
Does Google consider Forum Threads as Thin Content? Probably
Two threads (caveat: before 2015) suggest that Google might treat forum discussion threads as thin content.
Your two options here are either to noindex the entire forum (so Google doesn't see the threads) or move the forum to a subdomain (eg forums.yourwebsite.com). Even so, you might have to noindex the forums if you do get hit by thin content penalty eventually.
But wait, why even keep the threads indexable?
Discussion threads are great for super long-tail keywords.
When someone asks a question in your forum, they're likely to also ask Google that. Your discussion threads have some search value, especially for specific, niche queries.
The two sources I cited came from forums - Moz and Search Console specifically. If any site has great search practices, it's them.
Letting your forums be indexed can bring you long-tail search traffic.
To mitigate any thin content penalties, put your forums on a subdomain
And you definitely need to noindex certain pages with duplicate/similar content, such as user profiles, reply forms and so on. Then, as an additional bonus, if you notice a particular thread getting a lot of traffic and activity -- time to write it up as a blogpost.
There is also an unanswered query from here that never got a substantive reply: Could adding a forum hurt SEO?