With iOS7, it’s now quite easy to block someone from being able to message, call, or FaceTime you on iPhone and iPad, but things get harder when you ever decide you don't want someone blocked any longer.
Here’s the instruction ( both written and illustrated) on how to remove someone from your block list on iOS7:
Navigate to the Settings app from the Home screen of your iPhone or iPad and select either Messages, Phone, or FaceTime - any of these three work the same way. Once selected, scroll down and tap on Blocked. You’ll find Edit in the upper right hand corner, tap on it and then tap on red delete buttons that appear next to the person's email or phone number you'd like to unblock.
The final step is to confirm and tap on the Unblock button that appears. It’s all that simple. Despite the easiness, the nice thing about the Block List in iOS is that you can have only a certain email address or phone numbers for a person blocked and therefore, remove only certain ones if you’d like.
Check the instructions visualized on the image provided below.