If your only method of communication is a blockchain, you can't know who owns which public key. To know that you would need to talk directly to that person and

If your only method of communication is a blockchain, you can't know who owns which public key. To know that you would need to talk directly to that person and have them tell you "key such-and-such is mine". You still need that trusted channel. Since you can't know who controls the public key your messaging over the blockchain and they can't know the public key that's messaging them is controlled by you, an impersonation attack is trivial. Someone else can send that public key a message saying "hello, I am a1369209993, let's agree on a symmetric key". So, no, a blockchain is not a trusted channel. For the purposes of communication, it offers no more security than the public Internet. It doesn't even guarantee delivery of messages.