Well looks like I was an ID10T on this one. npm install pulls the original Annihil version cuz that is where package.json tells it to grab it from.
Manual installation of Julian's/NodeBB 3rd Party forked version, available here: https://github.com/NodeBB-Community/nodebb-plugin-email-blacklist
Coupled with my URL query string update above and ... voila! Now works dandy w/both manually added addresses and istempmail.org API key.
The function isTempMail's path var needs to be thusly:
path: '/api/check/Your-API-Key-Here/' + email
Or maybe I neglected the trailing slash first time around. In any event, geronimo, there ya' go.