Hi Patrycja,
I don't think reverse sorting really makes sense in the forums. The blog is different because the comments are about the blog post which is always above the comments, each comment may be independent of the others so seeing the newest ones first is a reasonable option.
A forum thread has the question in the first post of the thread and subsequent posts are in response to the question/first post on the thread. It would not make sense to have the answers before the question when reading a forum thread, especially on a long thread with lots of replies. It should stay in discussion order to have a meaningful context.
Note that when replying to a forum post the thread is sorted in reverse order beneath the editor so that users will see the most recent response before posting a new response.
Best,
Joe