So, I dont think its currently possible unless im missing something but boy oh boy would it be great to allow current site owners to switch the blog template.
What do I mean? Well, currently admins can of course switch themes but wouldn't it be great to allow them to only pick themes that are part of Blog templates?
In other words, not only would a new user be able to select a blog template at signup, but would be able to change their current blog to a new template. Hopefully that makes sense.