Short answer: It depends on the platform, but generally, no —messages sent while you were blocked will not come through after you unblock someone. However, future messages will be delivered normally.
So if you unblock an ex, a former friend, or a persistent salesperson, you won’t receive a flood of old messages. You’ll simply start fresh from that moment onward. Have you ever unblocked someone and missed an important message? Share your experience in the comments (or don’t—your block list is your business). when you unblock do messages come through