Both are basically content managers. A blog is content (the article) + content (comments on it). A forum is content (a thread & starting post) + content (the reply posts).
The difference is how the content appears and how it's generated. In a blog, the control and presentation of the content is primarily for "authors", a small set of dedicated writers, often just the person who made the blog. The comments are just supplementary; other people can respond to the blog post, but it's not the core focus.
In a forum, it's more about the discussion. A post at the top of the thread "ranks" about the same in terms of visual importance as a reply post, and there are more features to encourage discussion, and everyone and their content visually ranks the same (host, moderator, or newbie alike all get the same post interface).
Some people don't want to discuss stuff, they just want to read a centralized, easily siftable set of articles. Some people want to discuss more. Forums are bad for the former (good post content can be strewn throughout threads and threads can randomly be gold or junk); and blogs are bad for the latter (usually clumsy interface, visually de-emphasized and compressed at the bottom of an article).
So I think it's good to have both to cater to both groups. It's not too difficult to combine the two into one website, since both are based on LAMP stack.