Greetings All
Compared with the Mexican Ducks I've seen, this bird is as dark as the darkest, but has a rather pale face with a weak eyestripe (darker in Mexi) and a paler crown that that of Mexi Duck. It also has a barely discernible trailing edge to the speculum.
American Black Duck should have some darker markings on the underwing coverts, probably a different colored bill, a more purple speculum, less contrasting (darker, more streaked) face, and darker eyeline and crown.
I've sent these images to friends in LA and TX, and will post their opinions when received
Good Birding
Steve Mlodinow
Longmont CO