In the East of the US, female American Black Ducks are hard to distinquish from female Mallards.
However, the Mallard’s speculum (colorful iridescent patch on the secondary flight feathers) is blue, with a thin line of white along both its top and bottom edges, where the American Black Duck's speculum has no white, and ranges from a deep, dusky blue to purplish.

Central Park, Manhattan, New York City, New York, USA, 2 March 2007