After you block someone, you'll not be able to receive any messages from that person.
However, if you wish to change you mind and want to unblock that user, this is what you can do:
1. Click on that user's username.
2. This will take you to the user's profile.
3. On the screen, below their points, you'll find a option to "unblock" that user.
4. Click that button.
Hope it helps :)