With or without recursion, Quad9 reports NXDOMAIN for a blocked entry:
$ dig +nord @9.9.9.9 isitblocked.org A
; <<>> DiG 9.10.3-P4-Ubuntu <<>> +nord @9.9.9.9 isitblocked.org A
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 50942
;; flags: qr ad; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;isitblocked.org. IN A
;; Query time: 19 msec
;; SERVER: 9.9.9.9#53(9.9.9.9)
;; WHEN: Fri Dec 15 09:12:25 EST 2017
;; MSG SIZE rcvd: 44
$ dig @9.9.9.9 isitblocked.org A
; <<>> DiG 9.10.3-P4-Ubuntu <<>> @9.9.9.9 isitblocked.org A
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 8982
;; flags: qr rd ad; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;isitblocked.org. IN A
;; Query time: 12 msec
;; SERVER: 9.9.9.9#53(9.9.9.9)
;; WHEN: Fri Dec 15 09:12:42 EST 2017
;; MSG SIZE rcvd: 44
For a genuine non-existent domain, with recursion Quad9 hands off the query ('ra' flag set)
$ dig @9.9.9.9 isitblocked.org.asdf A
; <<>> DiG 9.10.3-P4-Ubuntu <<>> @9.9.9.9 isitblocked.org.asdf A
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 33765
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;isitblocked.org.asdf. IN A
;; AUTHORITY SECTION:
. 3162 IN SOA a.root-servers.net. nstld.verisign-grs.com. 2017121500 1800 900 604800 86400
;; Query time: 16 msec
;; SERVER: 9.9.9.9#53(9.9.9.9)
;; WHEN: Fri Dec 15 09:13:48 EST 2017
;; MSG SIZE rcvd: 124
Without recursion it returns REFUSED
$ dig +nord @9.9.9.9 isitblocked.org.asdf A
; <<>> DiG 9.10.3-P4-Ubuntu <<>> +nord @9.9.9.9 isitblocked.org.asdf A
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 53746
;; flags: qr ad; QUERY: 0, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; Query time: 20 msec
;; SERVER: 9.9.9.9#53(9.9.9.9)
;; WHEN: Fri Dec 15 09:13:43 EST 2017
;; MSG SIZE rcvd: 12
So you do not have to compare two different name servers to see if Quad9 is blocking or not.
Kudos to the president of IvyDNS for telling me to look at something along these lines.