Click the "Start" button on your computer and hover over "All Programs." Click "Google Talk" to open the program. You can also select Google Talk from the icon on your computer screen.
Now, enter your Gmail email ID and password to sign into Google Talk.
Click the “Setting” tab at the top right corner of the main page of Google Talk.
Once a separate screen appears, click “Blocked” tab located on the left panel of your screen.
A list of people you have currently blocked on your Google Talk will be open in front of you. In addition to that, their email addresses along with their user names will be also displayed to you.
Now, it is up to you; if you want to unblock someone, just highlight his or her name and click the “Unblock” option on the right side of the screen.
Once your are done with the task, hit the “OK” button at the bottom of the window to return to Google Talk screen. Enjoy your Google chat.