If someone has blocked, you on Fortnite you will not be able to message them / Whisper them.
You’ll be removed from their friends list.
But you can still match them in random games.
Your still able to match people that you have blocked or that have blocked you as if this wasn’t the case this could be used to take advantage in the competitive cups by blocking all the pro’s so you never face them.
You’ll still be able to see messages they type in the random games you match up in. But it won’t come up as their username.