They figure it out after trying to post on any of your non-anonymous questions, or your any of your opinions, or tries to upvote/downvote one of your opinions.
Or if they try to upvote your answer as an anonymous question asker. Because that doesn't work either if they're blocked.
Most Helpful Girl
They can only tell if they try to answer any of your non-anonymous questions or opinions or if they try to upvote one of your posts. That's the only way to know. There is no such notification that can let you know when someone blocks you as far as I know.
No, I don't think there is any notification that will let them know that they are blocked, however they will no longer be able to reply to your questions and they can't comment for your opinions as well, they can't message you as well. for example you blocked someone then when the person tries to comment on your opinion or tries to message you for that matter , then there will be a pop up which will let the person know that you blocked them.
They won't receive a notification. However, they'll realize it if they try to comment on your opinion because they won't be able to submit a comment. The same goes with someone trying to message you that has been blocked.