Since the beginning of the Google Custom Domain product, the ability to publish a blog to "mydomain.com", and optionally have Blogger associate "www.mydomain.com" with "mydomain.com", has been an assumed feature in custom domain publishing. Likewise, the ability to publish to "blog.mydomain.com", and optionally have "www.blog.mydomain.com" as an association, was an assumed, if slightly less popular option.
Recently, I discovered an oddity in the association. If you were to publish your blog to "blog.mydomain.com", you would get the option to "Redirect mydomain.com to blog.mydomain.com". In cases where there was another Blogger blog published to "mydomain.com", this would produce a disruption in the domain content.
Oddly enough, if you were to publish to "www.blog.mydomain.com", you would still get the option to "Redirect blog.mydomain.com to www.blog.mydomain.com".
I don't think that this confusion is coincidental to the recent addition of two level Top Level Domains to the custom domain publishing support structure.
Last week, in considering this oddity, and discovering that the need to publish to "blog.mydomain.com" is relatively unpopular, Blogger decided to discontinue support for aliasing non root custom domain URLs. This means that both "blog.mydomain.com" and "www.blog.mydomain.com" are now completely separate URLs, with no aliasing possible.
Right now, if you were to publish "bloga.blogspot.com" to "www.mydomain.com", then publish "blogb.blogspot.com" to "blog.mydomain.com", you may find the option to "Redirect mydomain.com to blog.mydomain.com" - in spite of having previously selected "Redirect mydomain.com to www.mydomain.com". If this is the case for your domain, then you may have to revisit the Publishing settings for "bloga.blogspot.com", and again select "Redirect mydomain.com to www.mydomain.com".