I might be missing something about the protocol, but when logged into BlueSky, I can't interact with BlackSky accounts at all. I specifically have to have an account there to even follow a BlackSky account.
I'm not as familiar with ATProto as ActivityPub, but following someone from another Mastodon instance, for example, is seamless as long as I'm logged in to the account I have on my home instance.
I follow, and am followed by, folks who use Blacksky and interact with them regularly. I have had zero issue with this, at any point. As Paul (the CTO of Bluesky) said in a sibling comment this would be a very serious bug.
FWIW— I have also not heard anything even remotely close to this at all from anyone using either service.
Can you describe what happens? That would be a pretty significant bug. Users are hosted all over the place and typically interact fine.