What to KnowSend the person a message. If it goes through, they probably haven't blocked you.If you see a warning saying that the message wasn't sent, the person might have blocked you.If you can view the person's Facebook profile, they may have blocked you on Messenger but not on Facebook.
This article explains how to tell if someone blocked you on Facebook Messenger with instructions for the desktop website and the mobile app.
How to Tell If Someone Blocked You on Messenger: Mobile Version
The easiest way to check whether you've been blocked on Messenger but not on Facebook is to use the mobile app and check whether or not a message gets through.
If it doesn't, you can then check if that person is still on Facebook. If they are, then they've blocked you on Messenger only.
While in the Messenger app, tap the Search bar and type in your friend's name.
Tap your friend's name when it appears in the search results.
Type your message in the text box near the bottom of the screen and select the send button.
If the message sends as normal, your friend hasn't blocked you on Messenger. But, if you're told "Message Not Sent" and that "This person isn't receiving messages at this time" this means either:You've been blocked on Messenger but not Facebook.You've been blocked on Facebook itself.Your friend has deactivated their account.
There's also a possibility that you won't get a message at all. The intended recipient will not receive your message nor be able to respond, though. So if you don't receive a response, it's possible you've been blocked.
In any case, your next step is to determine which of these possibilities apply. Open the Facebook app and conduct a search for your friend's name. If they appear in the search results after typing their name, then they might have blocked you on Facebook Messenger, but not on Facebook. But if your friend's account doesn't appear, this doesn't necessarily mean they've also blocked you on Facebook. They might have deactivated their account.
How to Tell If Someone Blocked You on Messenger: Desktop Version
The same basic methods apply when using your computer to check whether someone blocked you on Messenger, although the steps are slightly different.
Go to messenger.com and log in to your Facebook account.
Select the New Message icon in the top-right corner of the left-hand column.
Type the person's name into the search bar and select it once it appears.
Type a message in the conversation box.
Press the send button (arrow icon).
After clicking the Send button, you might receive a message which reads, "This person isn't available at the moment." Once again, this doesn't necessarily mean they've blocked you on Messenger since they could've either blocked you on Facebook or deactivated their account.
There's also a chance you'll see nothing out of the ordinary (as in the screenshot above), but the recipient will not receive your message nor be able to respond.