As part of alpha test for my new BLOG, which at present is Private, I opened a new WP account for my wife on her computer. She then requested access to my BLOG, I received the message via WP and allowed her access. She in turn received an Email from WP confirming access with a link to http://thirdtest.wordpress.com.
What has happened is that you have dwleted the “Hello Word” placeholder post and have not yet published an actual post. When you do publish a post the 404 will disappear. http://en.support.wordpress.com/not-found/