Apex domains cannot have CNAME records.
Question and response
Question
dig bluemarble.net. NS
Answer
bluemarble.net. 5 NS ns2.bluemarble.net.
bluemarble.net. 5 NS as2.bluemarble.net.
bluemarble.net. 5 NS as1.bluemarble.net.
bluemarble.net. 5 NS ns1.bluemarble.net.
Authority
The response has no authority records.
Additional
ns1.bluemarble.net. 5 A 66.244.64.229
ns2.bluemarble.net. 5 A 66.244.119.195
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0