The block list is just a table in Postgres with hashes of banned user IDs or post IDs. There's a Google Cloud Function to query it with an API key. The Planetary iOS app then checks the hash of each message and author ID against the ban list before writing it to its own SQL database. Here's the iOS code: https://github.com/planetary-social/planetary-ios/blob/main/Source/GoBot/GoBot.swift#L1230