You may one day log onto Facebook and see that one of your friends names isn’t clickable and no profile photo is visible, but you can still see the messages you exchanged.  This is a case where a person has deactivated their Facebook account, but hasn’t actually blocked you.

Being blocked on Facebook or account closed difference

If a person were to block you, you would not see the previous messages you exchanged, and you also would not see their name or profile photo.   You would see a thumb with a bandage if you were to try and pull up their Facebook profile page.