Good idea! That could definitely work. This seems like it could only be implemented client-side. It also seems like it would require rapid and frequent updates

Good idea! That could definitely work. This seems like it could only be implemented client-side. It also seems like it would require rapid and frequent updates to the ever-growing blacklist. That in turn would require a way to figure out what hashes needed to be blacklisted, which implies either less-than-ideal cryptography practices or the ability to decrypt messages. Neither is desirable for user privacy. I have a rule I use in computing: whenever you think you want a blacklist, you probably want a whitelist.