Deleting a free hosted WordPress.com blog requires ignoring multiple warning messages clearly stating that URL deletion is permanent and irreversible. Not only that it requites using the verification link sent in the email and ignoring that same warning there too.
The deleted blog URL will not become available again. I’m sorry but you cannot recover or register any deleted blog URL. 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/