I have three WP blogs, one of which has a unique domain name mapping. Neither the mapped name nor the original blog name with the wordpress.com extension shows up on my other 2 blogs. I would move the original to one of the other, but fear that I would lose my domain name mapping, especially since the mapped domain is very similar to one of the others. I have moved the basic content, but lost the original posting dates on the articles moved – they are not many.
When you register a username account here all that’s required is an email address, which becomes your unique identifier. If you are logged in and register a blog then it’s registered under that unique identifier.