Duck ID, Northern NJ, Today - I give up

I've looked on my eBird app, and in every guidebook I own. I'm at a loss. Please help. I can't figure out who this black and white duck is. She (I assume) was clearly paired with this male Mallard. They were with 3 other Mallard pairs and one pair of Canada Geese on a small pond in Hackettstown, NJ (Northwest part of the State).

Thanks so much. Please hurry with your replies; the suspense is killing me.

After poking around the net - my latest theory is maybe it's a Swedish Blue Duck x Mallard hybrid?

Malards are known for interbreding with any other duck. The result can be some odd combintions, with offspring not looking much like either parent.
To me, the white breast suggests a shoveler in the heritage somewhere..
I saw some very weird shoveler cross-breeds in Bosque; I'l post some eventually.

Sandy

generally when a mallard shows white in the plumage it reflects breeding with domestic ducks.

Just a mallard x domestic cross. Nothing more. All domestic duck "breeds" or "varieties" are mallards (Anas platyrhynchos). The only exception is domesticated Muscovy ducks. They are in a separate family altogether.