1. Why you might still see it
In certain situations (old messages, notifications, links), the username may appear. You can't click on it to access the profile, but Facebook keeps the reference for data consistency.
2. Old private messages
If you've exchanged Messenger messages, the username may remain visible in the conversation. You can no longer send new messages, but the history isn't deleted.
3. Past comments and tags
If the person has already tagged you or commented on your posts, their username may still be displayed. Blocking doesn't erase the past; it only prevents future interactions.