That's an interesting idea—adding specific notes for blocked users could improve transparency. Right now, our API doesn’t include a way to retrieve notes from blocked users, but it's something that could be considered in the future. That said, we’re very selective about blocking, and it’s mostly limited to spambots and impersonators of big accounts. Our API isn’t currently part of the Nostr protocol, and I’m not sure integrating it into clients would be the best approach since we already have NIP-56 for client-side filtering if needed. That gives users more control over what they want to see without relying on a centralized blocking system.

I don't think there is any need to actually retrieve notes for the user, especially for relays that are more picky than you. If a relay owner hates a user or their content, then making that relay store or retrieve notes from that user doesn't seem right to me. If I was defining a standard for relay filter transparency, it would only ask that a relay store the ID of the excluded post, the reason for excluding the post, and the relay that the post was obtained from. When I talk about developers implementing something like your API, I'm not talking about the user being able to copy your filters, I'm talking about users having the ability to more easily audit relays without ever having to leave their client.