Learn how to allow someone you blocked to contact you again from Facebook Messenger today.
Steps
Method 1 of 3: Unblock someone on iPhone and iPad

Step 1. Open the Facebook Messenger app
It is represented by a blue tooltip in the middle of which is a white lightning bolt.

Step 2. Tap the profile icon
It's a blue person icon that can be found in the upper left corner of the screen.

Step 3. Choose Contacts
This option is at the bottom of Notifications.

Step 4. Tap Blocked
You will see this option at the bottom of the page Contacts.

Step 5. Click on the name of the person you want to unblock

Step 6. Tap Unblock on Facebook
Then press Unblock. You can now trade with this person and vice versa.
Method 2 of 3: Unblock someone on Android

Step 1. Open the Facebook Messenger app
It is represented by a blue tooltip in the middle of which is a white lightning bolt.

Step 2. Tap the profile icon
It's a gray person icon located in the upper right corner of the screen.

Step 3. Scroll down and tap Contacts
You will see this option below Text.

Step 4. Tap Blocked People
This option is the last on the page Contacts.

Step 5. Select Unblock
This option is in front of the person's name.

Step 6. Choose Unblock on Messenger
This is the first option that appears in the window. When you tap it, you can chat with that person again through Facebook Messenger.
Method 3 of 3: Unblock someone from a computer

Step 1. Go to Facebook from your browser
Log into your Facebook account as needed

Step 2. Click the Down Arrow
It is located in the upper right corner of the screen.

Step 3. Select Settings
This option is in the lower half of the options menu.

Step 4. Choose Blocking
You will find this feature in the options menu on the left side of the page. It sits at the top half of the list.

Step 5. Scroll down to the Block messages section
The names you will find in this section are the names of all the people you have blocked from contacting you on Messenger.

Step 6. Click Unblock
This option is in front of the person's name. Make sure the name appears to the right of Block messages from. After that, you will now be able to message each other through Facebook Messenger.