Will they know I blocked them on Facebook?
Blocking is confidential, which means that users are not notified when they are blocked and people cannot distinguish between someone that blocked them and someone that deleted his own account.
However, if the blocked person can see that your profile is still visible from one of your friend's accounts or by people who are not Facebook users, it will be easy for this person to deduce that you didn't delete your account, and that therefore he, or she, was blocked by you.
