If you block them, they cannot see you at all.

However, if you are in a group discussion on a friend's FB page, they will see a sort of "blank area" where your post would be in the thread. I think what they see is your name in grey, but can't see your picture or anything else. I think that's how it works. So I kind of believe at least a few of these people have figured out, over time, that they are blocked by me. They were never on my friend list though.