Kenyon wrote that he was told that the term "Black Mail" came from the armor used in Medieval times. The armor which was worn was called maol and it became black (as described in black market). When the two knights were dueling and one attained the upperhand, he would give the other an alternative of life or death as the sword was pointed at his face. This was known as black mailing someone.
Andy in Scotland wrote: Centuries ago, Maol was the silver which was paid in rent in Scotland. Sheep and cattle stealers would steal the cattle and then try to legitimize the theft by threatening to keep the animals unless the owner paid them rent for the grazing the animals had while they were stolen. This became known as Black Maol or Blackmail. We Scots invented Blackmail, something to be proud of ??