I’ve been observing these ducks for a few weeks.  Beth said she thought the darker one was maybe a swedish blue duck, and the weird-looking mallard was some sort of hybrid.  I’m beginning to think that maybe they’re both hybrids.  The dark one resembles some pictures of swedish blue ducks, but not enough that I think it’s a match.  I saw them again this morning sleeping by a pond not far from the stream where I usually see them.  It’s interesting what you can find in your own back yard if you take the time to look around.