Although I enjoy the public key/private key ideas, If you wish to talk encrypted, one of the best ways to do such could be having signal if you don't mind centralization

But if you want decentralization some options i can recommend are matrix,simplex,session etc.

But to be honest, there is a good point that you raise about how to talk decentralized on bluesky

well, one of the ideas that I can think of right now, is that someone can use https://keyoxide.org/ and paste in their public key and also connect both bluesky and matrix and then have the keyoxide as part of something public like a comment

The problem in this is that it becomes tedious and does add more friction to the whole thing but definitely possible.