If you block someone on Facebook or Messenger, you both will not be able to see each others activities and also not be able to send messages. Old conversation will be still in inbox but name of that person will not be clickable.

See the Help Center to understand blocking:

What is blocking? What happens when I block someone?

You can block someone to unfriend them and prevent them from starting conversations with you or seeing things you post on your profile.

People you block can no longer:

• See things you post on your profile
• Tag you
• Invite you to events or groups
• Start a conversation with you
• Add you as a friend

Blocking is reciprocal, so you also won't be able to do things like start a conversation with them or add them as a friend. Keep in mind that blocking someone may not prevent all communications or interactions, such as in apps or groups, and only affects your experience on Facebook, not elsewhere on the web.

Note: Removing a block will not restore the friend connection. If you block a friend and then remove that person from your block list, you'll need to send them a new friend request.

• "you both will not be able to each others activities" … you both will not be able to what each others activities?

  – Kyle Delaney

  Commented Apr 13, 2021 at 23:50
• @KyleDelaney thanks for pointing this out. I have updated the answer.

  – serenesat

  Commented Apr 14, 2021 at 6:35

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• facebook-privacy
• facebook-messenger