I heard from the Cornell Lab yesterday, and they confirmed that these lovely birds that I posted about a few days ago are, indeed, the American Black Duck. It was a very nice email, and came with an invitation, which I'm sure that they extend to everyone, to join their e group to count the birds. I will do that.
The way to tell an American Black Duck from a female Mallard, according to Cornell is:
(The American Black duck will have) "Dark bodies, plain grayish faces and solid colored bills. Female Mallards would have mottled brown/orange bills, lighter and more patterned overall body color."