A blog is a web site. The two are the same thing. Every blog is an independent website. It matters not how many are registered under the same username account because deleting one blog has ZERO affect on any other blog.
You must be logged in under the exact same username account that registered a blog to delete the blog. To prevent accidental or inadvertent deletion of blogs, there are multiple warnings given that blog deletion is permanent and irreversible. Lastly, a verification link is sent in an email and must be clicked to delete a blog. If clicking that link does not work then copy and paste it into your upper browser window and click. http://en.support.wordpress.com/delete-site/
Deleted blog URLs are not recycled, regardless of who registered and deleted them and no exceptions are made. http://en.support.wordpress.com/recycling-blog-names/