Or do you mean delete a blog?
Provided you are logged in under the username account that registered the blog you can delete it by using these instructions. http://en.support.wordpress.com/delete-site/
Please note that deleted blog URLs are not recycled.
http://en.support.wordpress.com/recycling-blog-names/I do not recommend deleting the blog as it can be made private and used for testing and backup purposes. Using the privacy setting may be a better option > http://en.support.wordpress.com/settings/privacy-settings/
Hiding blogs is possible too > http://en.support.wordpress.com/my-blogs/#my-blogs-dashboard-visibility