Don't think you can... unless you re-add them to Messenger, then unblock them. Or start a conversation from the actions menu, and then try unblocking them.
Unblocking contacts - by Chancer on 05-24-2011 at 07:44 PM RE: Unblocking contacts - by whiz on 05-24-2011 at 08:23 PM RE: Unblocking contacts - by Chancer on 05-24-2011 at 09:15 PM