I'm no duck expert, so I hope I can answer this in a helpful (and relatively accurate) manner!!....IF anyone else out there can offer a better explanation (and/or correct any errors in mine), feel free to jump in!!!....
The standard Magpie duck is a breed. Colors are typically either black, blue or white. In the case of the black and the blue, the majority of the body "should be" white and there should be just splotches of color (black or blue) on the head, back, wings and tail. Unfortunately, the markings on hatchery and "utility" quality ducks often ends up being more black than white. I have magpies from two different hatcheries and most of them are in the "mostly black" category. They're still good and useful birds, but not necessarily the best for breeding IF you want to follow APA standards. (Both black and blue magpies can have solid white offspring, which, when bred to another white, produces all white offspring.)
Now, the Call Duck, in my understanding, is a breed in and of itself. It's sort of the "bantam" of the duck world. The APA recognizes several colors of Call ducks, including blue, buff, gray, pastel,snowy and white. As with a lot of other animals, certain breedings/crossings can result in other colors and traits. So there are other colors of the Call duck, including magpie. So, in the case of the call duck, magpie refers to the color, whereas in standard duck terms, the magpie is a breed.
I know!!...very confusing.
If you do plan to sell your ducklings and care about the quality, do yourself a favor and invest in a good quality breeding pair or trio. ( Check
http://www.holderreadfarm.com/index.htm . I've seen others recommend them in posts related to magpies. There may be other BYCers that have them availble too. ) If you want them just for eggs/meat, don't sweat the quality.