The system doesn't notify you when someone blocks on iMessage. You must look for signs and clues to confirm your blocked ...

Also, try the following methods to see if you have been blocked ... The first thing you can try is to send the person an iMessage. If you are not blocked, it will show as Delivered.

In case you don't see any indicator and instead notice ... messages still send in blue bubbles when trying to text someone who's got us blocked on iMessage, but without the "Delivered" indicator.