When someone blocks you on Facebook Messenger, you'll notice several clear indicators. First, you won't be able to send them messages—if you try, your message won't go through and you'll typically see an error or the message will fail to send. You also won't be able to see their profile, call them, or view their last seen status or online indicator. Any previous conversation thread with that person remains in your message list, but you cannot interact with it.
If you try to visit their profile directly, you may see a message indicating the profile is unavailable or that you don't have permission to view it. In some cases, they might appear completely removed from your friends list, though not always immediately. You won't receive notifications about their activity, and they won't appear in your contact suggestions.
One key distinction: being blocked is different from being unfriended. If someone unfriends you, you can still send them a message request (which they must accept), but if blocked, you cannot contact them at all through Messenger. The blocked person also won't be able to see your profile, messages, or activity.
It's worth noting that blocking can be temporary—the person may unblock you later. Additionally, if you suspect you're blocked, the most straightforward approach is to ask a mutual friend or try viewing the person's profile from a different account to confirm. Some people also deactivate or delete their accounts, which can look similar to being blocked from certain perspectives.