blog.mydomain.com. 3600 IN CNAME ghs.google.com.
www.blog.mydomain.com. 3600 IN CNAME ghs.google.com.
This is generally done when "mydomain.com" and / or "www/mydomain.com" is in use for hosting a web site. This is a great way to combine your blog and your web site. Yet there are other reasons to avoid publishing your blog to the domain root.
mydomain.com. 3600 IN A 184.108.40.206
mydomain.com. 3600 IN A 220.127.116.11
mydomain.com. 3600 IN A 18.104.22.168
mydomain.com. 3600 IN A 22.214.171.124
www.mydomain.com. 3600 IN CNAME ghs.google.com.
If your domain comes with content hosting (not just DNS hosting), the domain may be in use for hosting email, an FTP server, or miscellaneous content such as photos, on the domain server. Now, you can host your domain email with Google Apps, but FTP (hosted on "ftp.mydomain.com") and miscellaneous services will probably always be the responsibility of a third party host. And if you add the 4 x "A" referrals shown above, access to the third party host (besides explicit access to "ftp.mydomain.com") will stop working.
In these cases, your best choice is to publish the blog to a virtual host. Leave the FTP server, your photo library, and your web site, hosted on the third party server, at the domain root - and get started with weaving your blog into your domain.