French Black Copper Marans are the feathered legs original layer of dark brown eggs originated from Marans, France. Now hatcheries can be selling black Marans (kind of cheaper knock off) some are feathered legs some are not. I’m not sure what they cross bred them with but some will lay dark eggs and some will not. Not on standard, unlike if you can find reputable breeder of true FBCMs they are pretty strict on maintaining the quality birds, which normally you’ll pay more around $15-35 per chick and hatching eggs are also ranging from $5 to $15 an egg.
You are right on the cuckoos, they will only lay a shade or two darker eggs than regular brown eggs.
I got both cheaper Black Copper Marans and pricey ones from local breeder and I can tell the difference on their egg colors and quality of the birds and good feathering and temperament.