A page that lists all existing blogs alphabetically.
I can imagine this is *not* easy (large sites, multiple page-requests, caching problems, etc.)
Maybe it is possible to show a .. z with ten blogs from each letter and a link 'more...'
Once clicked on that link, a user will see first 50 blogs beginning with that letter. And page forward / backward (count* would not be server intensive).
This is something that I have never found. Some people tried but the need for caching and possiby cron jobs drives them away from wanting to develop this it seems.