There are several signs that can indicate that you have been blocked by a contact:

•  You no longer see the time of the contact's last visit or their online status in the chat window.
•  You no longer see updates to the contact's profile picture.
•  All your messages are marked with one check mark (sent message), but never with two (delivered message).
•  When creating a group, you can't add the person you think has blocked you.

However, the signs listed above do not give you 100% certainty that you are blacklisted by someone. Your friend may simply have changed their visibility settings.