When you setup your Google Custom Domain , the instruction for the DNS addressing setup is rather basic. Add one or more "A" and / or "CNAME" records. Point "www.mydomain.com", and / or "mydomain.com", to "ghs.google.com", or to a series of Google servers.  Unfortunately, all DNS hosts don't provide  multiple "A" referrals / "CNAME" referral to "ghs.google.com"; some only allow for a conventional "A" referral to a single, fixed IP address.   These instructions are most often seen when you purchase the domain directly from a third party registrar, and your registrar sets up the domain itself  - since when you use "Buy a domain", all of this detail is done for you.  
What Blogger won't tell you