Actually, blackmail has nothing to do with the Post Office, which screws up as a matter of principle. It comes from “mal,” Old English for tribute or rent. Warlords in ancient Scotland used to force farmers to pay mal as protection. Pay it and you plant your crops in peace; don’t pay it and we plant you and you rest in peace. Blackmail took on its modern meaning of general extortion for money in the 19th century.
Why black? The color was often used to suggest evil, but it may also have been to distinguish the payment, made in crops or livestock, from what was called “white” money --coins and currency.
Source: www.wordorigins.org
