This may be an example of inductive logic or maybe erroneous interprelation but here goes.
I've been spending a bit of time lately sharpening shit. If a tool has a cutting edge I've sharpened it over the last few days. Between Scary Sharp (tm), japanese waterstones, DMT diamond plates and the Tormek, I now have some very shiny and very sharp cutting tools - no fingerprints and dark stained hands.
It's the dark hands that got me thinking - Mafia- Black Hand - Assassins. Supposedly, the ultimate assassin is the one who gets close enough to kill with a knife. So I'm guessing that the knife they'd use would be sharp and to make sure it was they'd do their own sharpening. If one uses stones to sharpen with, one may wear one's fingerprints off - handy side benefit it one is an assassin. If oil is used in the sharpening process some stays on the hand(s), along with ground in stone grit.
Perhaps finger printless, dark stained hands were the mark of an assassin - sort of a status symbol. And perhaps finding a black hand print on your door would get you to associate it with assassins and put the fear of god in you - something the Mafia found useful in persuading people.
(I've been doing WAY too much sharpening.)
charlie b
ps - a Hock iron for a #7 takes freakin' forever to get the back flat.