Out of the box, this isn't possible with BE. You can do it of course if you use multiple websites or web applications.
But with the subdomains you have, it wouldn't be too difficult to achieve what you're looking for with a couple of modifications to BE.
Currently with BE, you can have multiple logins for bloggers -- so that's already taken care of.
You can modify BE to display a different theme/skin based on the subdomain. This is actually easy to do --
here's the basic idea of how to do it. This logic would probably be best put into the BlogBasePage file in the BE core (Web\Controls\BlogBasePage.cs).
BE does have the capability to display posts for any particular author or blogger. A modification can be made in the main default.aspx file to display posts by author depending on what subdomain the visitor has come to the site under.
Making these two changes might only require 30 minutes of time or less (once you have a feel for BE).