A blog is a site and deleting one blog has no affect on any other blogs registered under the same username account. That said, I recommend making the blog private and using it for testing and as a backup 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.
Deleted blog URLs are not recycled, regardless of who registered and deleted them and no exceptions are made.
To change blog visibility to private go to > Settings > Reading. See the guide here http://en.support.wordpress.com/settings/privacy-settings/