"Oh, my goodness, I have blocked the wrong person!"
This situation is often the case. No worries, however, as it is simple to correct. So, too, is Re-instating a Follower who has been removed from your circles.
Step 1 - The first thing you must do is navigate to the Profile of the person in question, which will appear like the following image.
Step 2 - Navigate to the bottom of the screen.
Step 3 - Click on the box which says 'BLOCKED".
Step 4 - Select a Circle to place him/her in.
NOTE: If a Follower has been 'Removed' from your Circles, he/she will need to be re-added to your Circles, as if he/she was a new Follower. Naturally, you can only Block, or Report, a Follower who is not a member of your Circles.