It is tricky but not impossible to tell whether or not someone has blocked you on Facebook Messenger. First, this information is shielded from you for privacy reasons. However, you may be able to work out someone has blocked you through the error messages that turn up when you try to message them. For example, a message that reads: “This person isn’t available right now” could signal you've been blocked on Messenger (or maybe completely blocked on Facebook). The methods explained above explain how to check further––just realize that none are foolproof and may mean something other than being blocked. You could ask the person through another channel or ask a mutual friend if you're game.