While putting together some useful resources regarding the use of blogs for existing online communities I found some interesting posts tackling this issue. Like here or here.

Most of the posts try to explain the differences between boards and blogs and there certainly are a lot of them. The main difference obviously is that a blog is far more ego-centered as it provides a personal space for the blogger to present his views and opinions. Somewhere I read (unfortunately I have forgotten where that was) this: compared to an entry in a messageboard a post on a blog is like standing up in front of a classroom and giving a presentation. 

Certainly both community tools have their advantages for certain use cases. That's why I believe that both will coexist and benefit each other. Participants of an online community might want to use a blog when publishing content that is linked to their personality and that they want to claim ownership for. Or when telling other members of the community more about themselves. Still they probably use forums for discussions or when looking for help on a certain issue. Especially with a singlesign-on blogs can be a perfect addition to existing online communities.