This is what the Online Etymology Dictionary says, 'blackmail (n.) 1550s, from black (adj.) + Middle English male"rent, tribute," from Old English mal "lawsuit, terms, bargaining, agreement," from Old Norsemal "speech, agreement;" related to Old Englishmæðel "meeting, council," mæl "speech," Gothicmaþl "meeting place," from Proto-Germanic*mathla-, from PIE *mod- "to meet, assemble" (seemeet (v.)). From the practice of freebooting clan chieftains who ran protection rackets against Scottish farmers. Black from the evil of the practice. Expanded c. 1826 to any type of extortion money. Compare silver mail "rent paid in money" (1590s); buttock-mail (Scottish, 1530s) "fine imposed for fornication."' http://www.etymonline.com/index.php?term=blackmail

Based on Ben's answer. Here is my guess: Black = negative, threatening Mail = mael = speech