Ive found that the best way to figure these things out is to breed them an see what you get
I think the Muscovys carry many different genes, and that the normal genetic rules don't always apply to them. I admit, I know very little about their genetics and how they work, but by breeding my own I have come up with colors people are saying I wouldn't get!
I have 3 that I use for breeding. They are all black and white pied, and one hen is barred. I ended up with chocolates (all were females), lavenders, whites, silvers, and black. Most were pied, and the few solid ones all molted out to have white heads.
The ducklings that ended up black I thought would be chocolate. As ducklings, they do look brownish. So maybe that's why the woman who sold them to you said they were all chocolate. They do look like a chocolate duck, but feather out black .
So even though both parents of the ducklings you bought were chocolate, I think the Muscovys have many recessive genes that can pop out in their offspring. I'm sure someone will correct me if I'm wrong! But like I said, from my experiences, the best way to find out what you can get by breeding your colors, is to breed them and hatch ducklings for a few clutches and see what you get. It is not always predictable!