When we block a contact in WhatsApp Messenger for Android, all these things happen:
- The blocked person will not be able to send us messages. We will not see their statuses either, as statuses are sent as a normal image in WhatsApp, and such sending is blocked.
- The person will not be able to send us files (or receive them from us).
- They will not see our profile picture, nor the last hour of connection.
- They will also not see our WhatsApp status.
- They will not be able to know if we are online or not.
- They will not be able to call us or make video calls with us.
However, even if we have blocked a contact we will still appear in their address book. This is something we can do nothing about; the only thing we can do about it is using the tools on the phone to block their calls and messages (just in case) and remove them from our address book. This way, when WhatsApp updates its list, the blocked person will no longer appear.
Needless to say, it is impossible to block a person on WhatsApp without them knowing about it. There are ways to check that someone has blocked us. As much as these forms are not 100% reliable, it does not mean that the contact will never find out, the person will sooner or later (probably sooner, almost immediately) realize what happened.
However, blocking a contact does not mean that we cannot write to them again in the future (as long as we have not deleted their number, or taken drastic measures when it comes to contacting us).