This is from wikipedia:
The word is derived from the word for tribute paid by English and Scottish border dwellers to Border Reivers in return for immunity from raids. This tribute was paid in goods or labour (reditus nigri, or "blackmail"): the opposite is blanche firmes or reditus albi, or "white rent" (denoting payment by silver).
In other words, it's pretty similar to paying for the local gangster so that he would leave you alone.