Hacker News Reader

I can't find it so I gotta ask: does Blue Sky integrates with mastodon instances and support ActivityPub?

Nope, AT Protocol is not compatible with ActivityPub.

But what are the differences? Does Blue Sky use another protocol just to be deliberately incompatible with existing federated networks?

Could mastodon instances theoretically be made compatible with AT Protocol anyway, and federate through both it and ActivityPub?