Of the confusion surrounding definition of Google Custom Domains in DNS, second only to the confusion about the DNS Referral definition, is the DNS Server definition.
Google does not provide the name servers needed for Google Custom Domains to work - the name servers must be provided by your registrar. The name servers are indicated by "NS" records, pointing to 2 - 8 servers owned by the registrar. You do not change the "NS" records.
The "NS" records point to the DNS servers provided by your DNS hosting company. You pay for the DNS hosting service, and the DNS hosting company sets up the "NS" records, on your behalf.
The name servers, provided by your registrar, are where the domain definition records, such as "A" (Address), "CNAME" (Canonical Name), and "MX" (Mail Exchange), among others, are stored. You have to provide a "CNAME" record on those servers, with that "CNAME" record pointing to "ghs.google.com", for your blog to be referenced as part of a Google Custom Domain.