_netblocks.blackboard.com has no CNAME records.
Question and response
Question
dig _netblocks.blackboard.com. NS
Answer
The response has no answer records.
Authority
blackboard.com. 867 SOA ns-719.awsdns-25.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0