mojoPortal can host any number of blogs but there is not anything currently to allow any authenticated user to have a blog automatically.
It would not be that hard to build since all the methods exist to manually create a page add a blog and give uer edit permission, it would just be writing code to automate this by calling the exisitng methods in correct sequence. One would need to implement a naming scheme for creating new pages to host the blogs and validation to avoid clashes with existing pages.
It would need to be implemented with a configuration switch to turn it off.
Also it would probably be better to make it require the user to request a blog rather than just create one automatically unless you assume that everyone who registers wants a blog. Seems better to have a "click here to create your blog" concept with maybe wizard steps to give the user some choices about things.