You just need to go to their profile page and click unblock. If you can't find the person, either search google for "name of person on ask.com," replacing "name of person" with their name and without quotes. If you can't find them for one reason or another, TOMus suggests this: "Go to privacy settings from the account tab on your homepage. Click on manage block list located under block list and select the person before clicking unblock next to the name."
Judging by the other comments, I guess it is not working well now, but that is how you "would" do it