You must be logged in under exact same username account that registered the blog and who is the Admin of the blog to access it and delete it.
To prevent accidental or inadvertent deletion of blogs, there are multiple warnings given that blog deletion is permanent. If you delete a blog you will not be able to get it back and no one else will be able to use that blog address. You must be sure you never want to use the blog again if you delete it.Lastly, a verification link is sent in an email and must be clicked to delete a blog. 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.
You may want to make the blog(s) private and use it as a backup and testing blog instead. http://en.support.wordpress.com/settings/privacy-settings/