I don't think complete invisibility is a good idea, but rendering as gray mannequins could be an alternative, similar to how others appear when you have the "Show friends only" option enabled

I like the idea, like on Black Mirror :) https://www.youtube.com/watch?v=AOHy4Ca9bkw

This is the thing. If such a feature is made it should have 2 tiers like Derender.

There are many practical uses for blocking. For instance as a veteran Giant Snail Racer we often relied on temporarily blocking our competitors while override blocking chat just so we can race without seeing their snail blocking the track. We unblock them after the race. I'm sure that's still practiced.

Removing annoying attachments and their hovertext that sometimes Display/Never won't eliminate for whatever reason also benefits from a temporary, non-malicious block that won't come back if you relog (as with Derender).

Therefore making this proposed ability an advanced block separate from Block Classic would be fantastic... and automatically add them to a property ban list on the parcel level.

The blocked person not seeing me (the blocker) is not something I would request, however I would like to be able to stop them from interacting with me. That means no pushing, no receiving IM:s, no receiving public chat, no receiving money transactions, no receiving items, no receiving Marketplace stuff. And it should include scripted or physical objects owned or controlled by them. Sometimes I'm happy with just blocking them partially though, and I would like to have that granularity in the blocking settings at the time of blocking another person: let me tick some boxes, and extra points for letting me add a comment (for memory, and perhaps a box for a personal auto-responder), and a block duration.

This could DEFINITELY be abused by trolls. They could block people around them to prevent them from seeing and reporting malicious activity in an area.

I've always wanted to be able to grief a region without the region owner knowing about it. Great idea.

No, I don't think this one-way invisibility is a good idea. It would allow people to sneak about on the victim's territory or in their presence– do we want a bunch of spies running around? It makes SL a nasty and confrontational world. Yes, there's a place for conflict, spy stories etc if that's what you want– in a Roleplay– but there, the interaction has been mutually agreed.

They have something like this in one of the other grids, if you block someone you both become invisible to each other and that's just that. Also invisible in the radar

This would amount to censoring the experience of others. This has been discussed before, rather recently even. The choice to block someone is a personal choice. What you suggest would impact their experience in a way they can't do anything about. While you might think "serves them right", this would constitute a far (over-)reaching impact on resident's experiences.