American Black Ducks can be found in both fresh and salt-water wetlands, often in the company of Mallards. Both species are primarily dabblers, feeding at or just below the surface, but on rare occasions, I've watched members of both species dive out of sight, surfacing moments later, presumably with a mouthful of food.
American Black Ducks have dark brown bodies with lighter brown heads. A male's bill is yellow, female's olive-green; both have black nails at the tip. The blueish wing patch in their secondaries -- called a speculum -- is bordered with black, unlike the white-bordered speculums of Mallards (the photo below highlights an American Black Duck in a crowd of Mallards). Hybrids of the two species (an example is pictured in Dabbling Ducks: Mallard) are fairly common, and these birds often show a mixture of plumage characteristics.