Not sure how this would work, but potentially one advantage of blockchain over peer-to-peer is less metadata needed?
All the encrypted messaged just get dumped into the public blockchain, you might be able to glean who (or at least where) a message was sent from, but the reader could be anyone with a decryption key. A bigger channel, like a newsletter type thing would just involve more read keys being available. Kind of like old school cold war numbers stations or encoded messages in the classifieds. You might be able to figure out that KGB agent Pyotr placed a classified ad for "Golden Lab, found near 5th Ave. and Main St." in the "lost and Fond" section, but there would be no way to know which of the papers many readers would be able to decode the message.
Of course the practical problem would be the size and scale of the blockchain. I think Bitcoin 1.0 was only able to do 7 transactions/second. With each message a transaction, and each read requiring the reader to pick out their message from the pile, my above hypothesis would have to be compromised in some fashion to be usable.