At FastMail, when we moved from our old in-house blog engine to Ghost(Pro), we moved from blog.fastmail.com to fastmail.blog, because the blog engine was being run by a third party, and if it was at blog.fastmail.com then we’d be giving our *.fastmail.com authentication cookies to said third party. So it was always going to have to be a different domain; then it’s just a matter of fastmail.blog being nicer than fastmailblog.com.