If the breeder you got the chicks from breeds Black, Black Copper, or Birchin Marans in addition to the Cuckoos then I would say they goofed and gave you the wrong chick. If they don't...
There are a lot of black chickens out there...lol Just check out the Black Chicken Show on the Online Poultry Show Forum. I would check with who you got the chicks from and if it turns out the goofed then you know what you have. Also find out any other black chickens they raise so if they don't have one of the Marans who are mostly black we have something to go on. I know Cuckoo flocks have thrown white sports...I wonder if they can throw black? Mine never did either...