You can’t block people. Who would you know, who registered the domain?
What you’re proposing is pretty similar to the current state of email. It’s almost impossible to set up your own small mail server and have it communicate the “mailiverse” since everyone will just assume you’re spam. And that lead to a situation where 99% of people are with one of the huge mail providers.
It’s extremely complicated and I don’t really see a solution.
You’d need gigantic resources and trust in those resources to vet accounts, comments, instances. Or very in depth verification processes, which in turn would limit privacy.
What I actually found interesting was bluesky’s invite system. Each user got a limited number of invite links and if a certain amount of your invitees were banned, you’d be banned/flagged to. That creates a web of trust, but of course also makes anonymous accounts impossible.
