I really like the idea of being able to block someone's PMs like this, but I'm not sure I agree with how it was implemented.

Hush, filter, and mute have a good thing going for them, they are ACL based and you can individually choose which features for each user you wish to apply to different users. The way that block is worded:

block: hush, filter, mute, and {user} and I cannot PM or reply to each other

Makes it a little less consistent in design terms. If I wish to for example, block a person's PMs because they are just being annoying and flooding me with messages, but in general are a decent person, that should be something you should be able to block without hush/filter/muting them. Also I think there might be instances where you in fact disagree with someone strongly and want to hush, mute and block, but not filter their posts because they might actually share interesting things.

These are just two examples off the top of my head, but the simplicity of the design of ACLs in general are better practice IMHO. It gives more power to the user that way.

So my suggestion would be:

hush: sort {user}'s comments to the bottom of threads

filter: remove {user}'s posts from my feeds

mute: {user} cannot comment on my posts

block: {user} and I cannot PM or reply to each other

That way if someone wants to block, mute, filter, and hush someone, they just check all four boxes. It doesn't remove any functionality of the new feature, it just adds some granularity to the feature and keeps the existing features and design consistent on the site.