Since the beginning of the Google Custom Domain option, the ability to publish a blog to "www.mydomain.com", and optionally have Blogger associate "mydomain.com" with "www.mydomain.com", has been an assumed feature in custom domain publishing.
Similarly, 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.
Long ago, I discovered an oddity in the association, previously accepted as a feature.
If you were to publish your blog to "www.blog.mydomain.com", you would get the option to "Redirect blog.mydomain.com to www.blog.mydomain.com". However, 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 already published to "www.mydomain.com", and "Redirect mydomain.com to www.mydomain.com" had been selected, this would produce a disruption in the domain content.
I don't think that this confusion is coincidental to the addition of two level Top Level Domains, to the custom domain publishing support structure.
Another disaster was observed when people intentionally publishing to "blog.mydomain.com" - or to "www.blog.mydomain.com" - were presented with the Redirect option. Naively deciding "Why not?", and selecting the redirect (even with no DNS address having been setup to for the aliased host), the blog owner would later see the infamous "Another blog ...." (now "Key already exists ...").
In 2010, when considering this potential for disaster, and oddity - and observing that the need to publish to "blog.mydomain.com" is relatively unpopular (at least 99% of all custom domains involve publishing to "www.mydomain.com") - 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".
My recommendation - if you want a stable domain - is that you publish to the "www" host - that you only alias the "www" host and domain root - and that you never combine a Blogger blog and a website, using the "www" host and domain root, together.