You and I have a secure P2P connection (aka Skype). But my client and your client check hashes of URLs against a third party service before transmitting the message to the other person.
There, you can blacklist URLs and it doesn't send any message contents to anyone (only hashes of ONLY urls) and then sends the actual full message, encrypted on the wire.
It's not hard to conceive how this system would work.
You and I have a secure P2P connection (aka Skype). But my client and your client check hashes of URLs against a third party service before transmitting the message to the other person.
There, you can blacklist URLs and it doesn't send any message contents to anyone (only hashes of ONLY urls) and then sends the actual full message, encrypted on the wire.
It's not hard to conceive how this system would work.