As far as I know, they can still see any messages you post in the public chat but they can’t direct message you anymore. 

They cant see anything

I mean if its on the app

In case someone stumbles here after a google search: when you block them and they try to send a message the server will return the message that the text was successfully sent but rejected by the recipient. When you remove them from the friends list the server will tell them that they are no longer your friend and invite them to send you another friend request in order to chat.

Bottom line, yes they will know.

This regards the app.