I have the Samsung SideKick.. I blocked a friend, now I can't "unblock" him.
all you have to do is touch and hold a message from them and choose "unblock". if you don't have a message from them, go back to the main messages screen, tap Menu button then Settings ... from there edit your Common Settings Block List.
you can check pages 110-111 if you get stuck http://support.t-mobile.com/docs/DOC-1753