Blocking a user behaves more like mute. Proper blocking needed to prevent harassment.
The block feature should be renamed to “mute”, which is what it seems to actually be. Currently I can apply this to a user and they can still see all my posts. So it’s a good mute feature but a terrible block feature.
Would a two way block be even practically possible on a network like this? Whatever server they are on would see your posts, so all it would take is for that server to use a slightly modified version that doesn't hide your posts to blocked users and they'd see them anyway.
You have to know that they're doing this though. Suppose some troll is self-hosting, or part of a very small instance? You'd only know they'd do this if they told you
A good two way blocking system should mainly focus on preventing interaction from the blocked user (The specs of Activitypub mention interraction, as opposed to viewing), even if they don't hide it, the interactions from the blocked user wouldn't be federated as if they were a banned user.