A quick note to myself the next time I stumble over this: when you instruct Blogger to map the blog to a 3rd party (custom) domain it basically instructs you to add three entries to your domain’s zone file:
1. a mapping from the domain to ghs.google.com
2. a mapping from some weird subdomain you’ll be given to another wreird subdomain at google. Apparently this is some load balancing measure
3. A TXT verification record
Now, these google domains you’ll be given are qualified (“absolute”) domains and always end with a dot. If your domain is example.com and you remove the dot:
blog 3600 IN CNAME ghs.google.com
(Note that ghs.google.com does not end with a dot)
Here “blog” will map to blog.ghs.google.com.example.com.
The correct zonefile entry is
blog 3600 IN CNAME ghs.google.com.