Much like users picking an instance to join because of a theme or the moderation policies, instances could join a federation because of a grand moderation policy.
If a CA is compromised, which they have and they will, then the trust is broken at the root and anything that inherited that trust needs to have it reestablished.
I’m not eager to apply blockchain to something that is a governance issue.
The general idea is trusting more than one authority, polling them for a consensus on which nodes are filtered/restricted. Many nodes would simply proxy their inherited filters but can change what node they would inherit trust from.
As far as I know, `domain-blocks` is as far as mastodon has gotten for inter-nodal moderation.
I remember seeing a list posted by [@Gargron](https://mastodon.social/@Gargron) of domains that had media restrictions placed on them but I can't find any mention of this in mastodon's documentation.