1 Messages will never be delivered to you
When you block someone on Whatsapp, their messages will never be sent to you. They will just see one tick beside the message but the second tick will never appear.
2 Your last seen will disappear
When you block someone on Whatsapp, they won't be able to tell when you last used Whatsapp because your last seen information will disappear from their Whatsapp.
3 You will have to unblock them to message them
To message someone you blocked you will have to unblock them first. If you try to send a message anyway, the iPhone will prompt you to unblock them.
4 They won't get a notification
When you block a person on Whatsapp they won't be getting any notifications. However, a person might realize he got blocked as he fails to deliver messages to you.
5 Your status message won't be seen
When you block someone, your status message won't be seen by them anymore. Your previous status will also disappear. (See What happens when you block someone on Instagram?)
6 Your profile picture won't be seen
When you block a person, your profile picture won't be seen by them. Your previous profile picture will disappear the moment you block them and the default Whatsapp picture will show up.
7 Contact information remains the same
When you block a person, both you and that person will retain the contact information on your phones.
8 You won't be able to call each other
When you block a person on Whatsapp, the ability to call each other using the app won't work for both of you until you unblock the person. In such a case, the calls won't go through. (See What happens when you block someone on Facebook?)