Apex domains cannot have CNAME records.
Question and response
Question
dig 1blu.de. NS
Answer
1blu.de. 5 NS ns1.1blu-dns.de.
1blu.de. 5 NS ns3.1blu-dns.net.
1blu.de. 5 NS ns2.1blu-dns.eu.
Authority
The response has no authority records.
Additional
ns1.1blu-dns.de. 5 A 87.238.196.21
ns2.1blu-dns.eu. 5 A 178.254.5.21
ns3.1blu-dns.net. 5 A 178.254.63.21
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0