James, they are beautiful! You are right, your black ones do look similar to one of ours, but they are twice as big! I am quite sure now that ours is some variety of a call duck.
We thought long and hard if we'd let the ducks in our pond and decided against it. They would terminate any wildlife in there in a day and I do love my frogs, toads and newts. We have given them the kids' old sandpit to play in and they seem to be happy with that. It can be easily cleaned, which is important, as they poo a lot in the water. After only a few weeks they had turned their enclosure into a smelly mud puddle and they were so unhappy, always looking at the grass outside - so they now have the former lawn/new orchard to roam in. They do eat the grass, but also dig it over and eat the worms and roots underneath, which shows mainly at the edges of the area. I will fence some parts off and reseed the destroyed areas. I agree with Annie, that ducks can thrive if you give them plenty of greens. I also give them some weeds from the vegetable patch, they love ground elder and chick weed.