blog.server.com has no NS records.
NS records for this zone are defined at server.com.
NS records for this zone are defined at server.com.
Question and response
Question
dig blog.server.com. NS
Answer
The response has no answer records.
Authority
server.com. 1800 SOA kenia.ns.cloudflare.com. dns.cloudflare.com. 2395565675 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0