shouldn't be blacklisted, needed for 2FA

Queries for are blocked which disables 2FA for American Express' mobile app required to verify online payments.
Hence I suggest to remove from the blacklist.

Pi-hole does not maintain any blocklists.

That is something you should communicate and discuss with the respective blocklist maintainers.

Thanks. Who would be these?

run pihole -q in the console and you will know which list is blocking it (or in the web interface via tools -> query lists).

Just ran it on mine and has it on the list.

(For what it's worth, haven't experienced any issues with AMEX so far.)