They don't have to be the same. A given user can have any number of blogs associated with his or her account. My plugin gives you control over the usernames, while WPMU Blog Name Restrictions Override gives you control over the blog names.
If you're a plugin developer, each of these plugins hooks into different parts of the WordPress code. WPMU Blog Name Restrictions Override uses
wpmu_validate_blog_signup while my plugin users
The two plugins work great together, and the fact that the options in mine are similar is no coincidence. As noted in the plugin description my plugin is based on WPMU Blog Name Restrictions Override.