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. Once clicked the blog URL is forever gone, gone, gone.
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.
You can register another .wordpress.com blog http://en.support.wordpress.com/register-a-blog/
Then if you wish to you can register a domain via a domain mapping upgrade because it does not matter what the underlying .wordpress.com address is when you are domain mapping.