And I guarantee at some point people are going to start thinking “Hey, if this instance isn’t subscribing to the shared blocklist, that must mean they want those other instances around, and therefore they deserve to be added to the list”

Already is happening on the Mastodon/Pleroma side of the Fediverse. Instances getting de-federated because they dared allowed their users to see posts from “bad” instances

Shared Fediblocks are a plague. Get added to one by somebody that personally dislikes you and every instance that uses the block will unknowingly block you.