Maybe it is I who have been chasing a wild goose. This BBC site (I've put the http:// in this time, Reinhard) says, "The Reivers, whilst having been some of the bloodiest characters in British history, have left a legacy of words in the English language. Terms in use today that stem directly from the reivers are Blackmail, Bereaved, Kidnap."Perhaps it's just that the first usage of the word "blackmail" was to describe one of the Reivers' nefarious activities.
Sorry.