Well, there is so much hybridization amongst the mallard complex, that the bird certainly may not be "pure" anything. In my opinion, I would say black duck. The reason is the bill color versus the speculum, or feather darkness. I have a Florida mottled that I would call a black duck if I had shot it here in Wisconsin. I have shot perhaps 10 black ducks over the years here in Wisconsin, and a couple in Massachusetts. I would say that the two in Massachusetts were mutts. Both had a hint of green in the head. At three feet, you could not see the color, but it was certainly there. The blacks we have here in Wisconsin, are very black for the most part, but a few are lighter, and the hens are different colored as well. The deciding factor for me, is if there are at least 14 brown feathers on the underside of the wing near the wrist, where the leading edge is.
Many years ago, there was a study being conducted in New York, to do DNA sampling on black ducks, to determine if there is actually a species that is pure any longer? I'm not sure which came first, the mallard, the black, the Florida, or Gulf Coast mottled, the Mexican, or the Hawaiian, but I'm pretty sure, all of them are a derivative of a mallard? I never heard anything after the article, but I bet the samples were all over the place. On the East coast, there are so many mutts, that are combinations of black and mallard, that where the two species of mottled and blacks may meet (perhaps even this bird) would certainly screw one another, and you would again have even further hybridization? I know this problem happens where the mottled, Mexican, and Hawaiian ducks interbreed with common mallards.