I'm still having trouble grasping the distinction between the two. Do they both have a similar backend structure, but just present the information two different ways? Both let users create their posts (forum/blog) where other people can leave replies/comments.
Is there a fundamental difference I'm missing? Or could I effectively mimic a Community Forum with custom templates on a Community Blog and vice versa?
Reported on Movable Type 4.2

I've been wondering exactly the same thing.
If an expert could chime in that would be grand:-)
So I did a little research with the DB layout and template sets. From what I learned there really is no structural difference from a community blog and community forum. It is taking the same data and presenting it in a different manner.