Most likely an infection or a lack of circulation. If your hand has turned black, you are probably too late but should IMMEDIATELY seek medical attention, LIKE NOW!
If the hand is lost, the gang green that will follow could easily set up sepsis in the body and kill you.
Gangrene due to lack of oxygen or frosbite. You can get frosbite by loosing warmth in your hand.
they turn blck because they need more lotion
Starch.
Rusting
Mold
Gravitation.
A broken car =]
Brake dust!
Bugs are wiping their asses on them
Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air.
Too much iron in the water
ur hand will probably turn black
It is the high temp and the high return Pressure inside the injector that causes the fuel To turn black
Smoking turns the teeth a yellow-black color because of the tar that is in it.