Here's how to unblock someone:
- Click 'Settings' at the top of your Friends list.
- Highlight 'Blocked' along the left side of the dialogue box.
- Find the person you'd like to unblock, and highlight his or her email address.
- Click 'Unblock,' and then click 'OK.'
Once you unblock someone, you can start talking again as long as the person hasn't blocked you, too.