I'm perfectly content to see federation whitelisting, blacklisting, and graylisting in the Mastodon ecosystem.
It's clear from the history of email that all three approaches are necessary:
* Blacklists for e.g. the known spammers.
* Whitelists for e.g. corporate email that needs to stay in-house.
* Graylists for e.g. Google Mail, which has to deal with everyone.
The tools for a wide spectrum of federation approaches are being built even now.