@thomasfuchs "respect for post deletion" if the post was later deleted by the poster or a moderator (on your instance or theirs) means that the server may not have actually retained the relevant post to show you for context. (And artificially retaining deleted posts for purposes other than instance-level moderation can also start getting into "right to be forgotten" territory in the EU to my understanding?)
But if the post hasn't been deleted or blocked by instance moderators, I don't see any particular reason why it couldn't record the URL of the post you were looking at when you blocked them, then retrieve that as a "context" link on the "user is blocked" thing.
And if that's "not an option", it would be great if the block/mute buttons in the context menu had a quick little "flow" with an entry box for the private note field so you can set that right then and there when you block them.