Just like you could register your own domain and return the record of a blacklisted website?

Of course, but presumably the censoring dns server would never return a censored ip, regardless of domain. Whereas if you were to passthrough all non-blacklisted queries you wouldn't be able to block that, which is why no censor would ever implement it that way.

Which is why you'd end up implementing tunneling for your DNS requests like cloudflared provides.