Deleting a URL is permanent and irreversible. 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/
Every blog has a sole owner and it is the person with the username account who registered the blog under that username account and associated email address. The blog's owner can transfer the blog to another person with a WordPress.com username account.
You must be logged in as Admin http://en.support.wordpress.com/user-roles/#administrator under the exact same username account that registered the blog and purchased upgrades to renew them or transfer the blogs and the upgrades to another username account.
To transfer a blog from one username account A to another username account B you must first log in under the username account A that owns the blog you want to transfer to the other username account.
Then you add the other username account B as an Admin of the blog.
adding users http://en.support.wordpress.com/adding-users/
user roles http://en.support.wordpress.com/user-roles/
Then you A can use this process to transfer the blog to that username account B instead http://en.support.wordpress.com/moving-a-blog/#transferring-your-blog-to-another-user-or-account