Do NOT ever delete a blog to get rid of the error message on the front page.
re: the front page error message
By default the front page of your blog displays all posts. The 404 (page not found) error message on the front page of the blog appeared because the Admin of the blog deleted the placeholder Hello World post. http://en.support.wordpress.com/not-found/ As soon as you publish a post, not a page it will disappear. Read the differences between Pages and Posts.
NOTE: This link to the step by step tutorial prepared by Staff is found on the bottom of your Admin page http://learn.wordpress.com
Deleting a blog URL is forever. Making the blog private is the better way to go instead.
Provided you are logged in under the exact same username account that registered the blog you can delete any blog registered under that username by using these instructions. http://en.support.wordpress.com/delete-site/
Note that deleted blog URLs are not recycled.
To make the blog visibility private and use it as a backup and/or test blog see http://en.support.wordpress.com/settings/privacy-settings/