I must tell you, that before I moved to Columbus and frequented the Walden waterfowl refuge at Blendon Woods, I had never heard of a "black duck". My introduction to waterfowl was at Blendon Woods, and each year, several hundred individuals winter on the pond that is kept ice free by several aerators that constantly move the water. The black duck looks similar to a mallard, but the males lack the bright green head, and overall, they have a very dark appearance.
The males and females are quite similar, but the best way to tell them apart is to exampine their bills. Males have yellow bills, while the female's bill is brownish.
In a few weeks the black ducks at Blendon Woods will fly north to their breeding grounds. I'm hoping a few more diving ducks join them before the waterfowl migration, now in full swing, is over.