I saw these black duck 8/15/2010 at Tifft Nature Preserve near Buffalo, NY. I thought I knew waterfowl pretty well, but this is stumping me! My guess is juvenile plumage of something?
all black duck??
Not a confirmation but thinking a dark version (over time) of domesticated Mallards. Another may be able to give an Id on these birds.
I cannot tell the size of the duck but it may be a Cayuga. If it is a large duck and the black feathers have a lovely green sheen then that is what you spotted. I also cannot tell for certain the sex but females will slowly change from black to white as they age. The males may show some white but not as much as the gals!