I think the most important elements of a blog are already available in the profile page, but needs some template edits.. to make it look like a real blog:
- Remove profile posts ( There is already a good add-on for that )
- Display the information tab like "About Me" widget
- Display the posting tab as Posts.. ( But show only Threads previews from Specific forums... only thread previews and not posts.)
Sure, this is not a real Blog System, but it might be more than sufficient for many admins.. and a better choice than the actual profile post tab...