I found this words as I was trawling through debunked etymology myths on snopes.com. The word blackmail seems like it would make sense if it were simply a combination of black (which was basically a synonym for evil back in the days) and mail, as in missives. This is not the case, for the latter, at least. As you may recall from my first ever post, the word black came from the Germanic word for "fire", which came from the PIE word for "burnt". The mail part also traces back to PIE, as mod, "to meet or assemble". This went into Proto-Germanic as mothla and Gothic as mapl, or "meeting place", kind of a metynomic shift (a metynomic shift is when something takes on the meaning of something previously associated with the former) and this became maedel in Old English, undergoing a double metynomic shift to mean "meeting" or "council". This then became mal, or "lawsuit" and male meaning "tribute". This, combined with black, utilized a sort of different spelling, and meant "evil tribute", and had nothing to do with "evil mail".
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd