Thank you.
$ grep -R email_domains_blocklist ./
doesn’t return anything
$ grep -R email_domains_blacklist ./
./script/import_scripts/base.rb: email_domains_blacklist: '',
./app/models/site_setting.rb: email_domains_blacklist: "blocked_email_domains",
returns as above. Meaning it’s most probably blocked_email_domains. OTOH I noticed that in previous attempts I replaced the original script/import_scripts/base.rb with the gist based one linked to above by @Paul_King:
I restored the original one back and it seems to have gone through this time. Too good to be true. I/It must have missed something 
Going to find out now