It’s helpful to understand what triggers notifications versus what doesn’t:
Here's what you should know about blocking and unblocking on Facebook: when you unblock someone on facebook does it notify them
Even without a notification, a user may realize they are no longer blocked if they: Unblock someone's profile on Messenger - Facebook when you unblock someone on facebook does it notify them
: You cannot re-block the same person for 48 hours after unblocking them. How They Might Find Out when you unblock someone on facebook does it notify them