We saw this duck last weekend at Bombay Hook. I was so interested in Shearness Pond I forgot to look behind me. My wife pointed it out to me and then was able to photograph it.
I did not notice the blue feathers until I zoomed in 200x. It shows up nicely in these pictures.
I especially like the second picture. I think it is posing.
I originally labeled this as an American Black Duck but my father-in-law pointed out that female Black Ducks have purple plumage in the wings. Mallards have the beautiful blue. My father-in-law is a tremendous wealth of birding knowledge and I trust his knowledge implicitly.