If you are trying to "unblock" a person on the list so that person can call you do the following: Open the "Skype" menu and click on "Privacy". You should see a button that says "Blocked Contacts". Click on that button and it will exposed a list of the callers you have blocked. You can delete any or all of these callers.
BTW, I know it works for Windows but I do not know if it works for Mac.