How to block or Unblock a contact on your Android phone:
To block a contact on your Android phone: Launch WhatsApp then go to your Chats screen.
Tap the Menu Button > Settings > Account > Privacy > Blocked Contacts. Long press block contacts and click on + user icon on the top right and add Whatsapp contact to block.
To Unblock simply go to > Whatsapp Settings > Account > Privacy > Blocked Contacts it will display the list of blocked contacts tab and that and tab on th user you want to Unblock.
Learn How to Block or unblock someone on iPhone.