mh.blackboard.com has no NS records.
NS records for this zone are defined at blackboard.com.
NS records for this zone are defined at blackboard.com.
Question and response
Question
dig mh.blackboard.com. NS
Answer
The response has no answer records.
Authority
blackboard.com. 900 SOA ns-719.awsdns-25.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0