_netblocks.google.com has no NS records.
NS records for this zone are defined at google.com.
NS records for this zone are defined at google.com.
Question and response
Question
dig _netblocks.google.com. NS
Answer
The response has no answer records.
Authority
google.com. 60 SOA ns1.google.com. dns-admin.google.com. 873285235 900 900 1800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0