You are most likely referring to a Black Swedish Duck. They are mostly black with a white breast. Their bills are traditionally a yellowish olive color.
Black Swedish Ducks are often referred to as the Daffy Duck breed.
That might be a Magpie. I have a Magpie Duck that is black and cream colored. She has a black cap on her head, black breast, and is black-spotted all down her back and wings, with orange feet/legs and bill. I've seen pictures of Magpie Ducks that are almost all black, with just a little white or cream on them.
Does she stand almost straight up at times? If so, then she may be a Magpie. They have Indian Runner in their ancestry, so at times, especially when curios, they'll stand almost straight up.
There's not much info on that breed, and from what I can tell it's a little rare in the US. You can show them, but there's no real standard for color, as breeders have found that they can't get the same results over and over.
