This should be pretty simple - prevent posts from loading if they are on an instance blacklist. This will allow users to avoid content from certain instances (such as Exploding Heads or Lemmygrad) that they do not enjoy, regardless of whether their instance ends up implementing a feature to do similar blocking.

It might be good to add in a user toggle (set to enabled by default) that allows comments from blacklisted instances to be shown on threads (to avoid bad formatting issues).