Apex domains cannot have CNAME records.
Question and response
Question
dig blackboard.com. NS
Answer
blackboard.com. 4 NS ns-1824.awsdns-36.co.uk.
blackboard.com. 4 NS ns-337.awsdns-42.com.
blackboard.com. 4 NS ns-719.awsdns-25.net.
blackboard.com. 4 NS ns-1321.awsdns-37.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0