To remove a player from your Black List, open their profile and click on the little man with the minus black symbol:

You will see the list of people you have put in the Black List, the reason for putting them on the Black List and a counter with the total number of people you have put on the Black List (in the following image there is only one 1/500)

To remove a player from the Black List click on the little man symbol with the red X (as indicated by the red arrow in the image below)

The following message box will appear:

if you are sure you want to remove it from your Blacklist click on YES