That is interesting
The text of the message (email) was
It is hard-coded into the blog.php page to not allow unregistered users to see the blog page (see around line 328).+ If you want to change this, this would require modifying the code (i.e., 'hacks', plugins, or template modifications).+ We can't offer official support for code modifications or for forums running modified code
This is quite important to me as although I want the forums to be closed and for my customers only I do want to be able to blog and not have to use third party blogging software - which is why I went for vbulletin to start with.
I'll have another look and see if I can work out what is going on. I wonder whether it is because the forum is locked down that the blogs are also. If you find an answer I would of course be interested