When you block a contact on WhatsApp, the blocked person is unable to interact with you over the messaging app. However, before you block a contact, or even after blocking, it is common to feel curious about – among other things – if the contact knows or would come to know that they've been blocked.

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Also, the blocked person can still contact you outside of the platform.
Here are six facts you should know about blocking a contact on WhatsApp, courtesy TechTudo:
(1) Message, photo, status and ‘Last seen’: Upon being blocked, a user's photo, status, ‘Last Seen’ and message, all disappear from the blocker's app.
Conversely, a user can also hide these items from someone, without necessarily blocking them. For this, press the three dots in the upper right corner of your WhatsApp chat, and then ‘Settings', ‘Account’ and ‘Privacy’.
(2.) Message after unblocking: Even after a contact is unblocked, any message that the person sent while they were barred will not be delivered. This is because during this period, the person was in a ‘limbo’. However, once unblocked, conversations can take place normally.
{{/usCountry}}(2.) Message after unblocking: Even after a contact is unblocked, any message that the person sent while they were barred will not be delivered. This is because during this period, the person was in a ‘limbo’. However, once unblocked, conversations can take place normally.
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(3.) Group messaging: If the blocked individual is a member of the same group as you, they will be able to exchange messages with you and tag you. Images, videos and audios shared in the group will be visible to them as well.
To prevent this communication, either leave the group yourself, or ask the admin to remove that individual.
(4.) What happens to contact name: The blocker's name will continue to appear on the blocked user's app and vice versa.
(5.) Blocking vs removing: When you block a contact, the blocked person will not be able to communicate with you through the app. On the other hand, if you remove a contact from your phone, the individual will still be able to call you, though only his phone number will appear on the call and not the name.
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Also, if the number is only removed from your phone book and not blocked on WhatsApp, the person will be able to contact you from the service.
(6.) Can a blocked contact call you on phone: Yes. If you don't want to receive a normal call as well, block the person's number on your phone book.