When you block someone on WhatsApp, they are not notified by the app. However, the blocked person may realize they are blocked because they can no longer see your "last seen," profile photo, status updates, or online status. They also cannot send you messages or call you; messages they send will show only a single tick (message sent but not delivered), and calls will not go through. Additionally, they will be unable to add you to groups or see changes you make to your profile. Despite these signs, WhatsApp does not send any direct notification informing the person that you have blocked them.