Breed a male with one copy of the gene with a cuckoo female and you will get males with one and two copies of the gene and both cuckoo and black females. Breed a male with one copy with a black female and you will get cuckoo (one copy) and black of both genders.
Yup, it most likely came from a cuckoo rooster. But it will not be able to pass the gene for cuckoo to its kids. You can still use it in your project, but it will be the same as useing any other black colored silkie in the project. No extra hidden advantage.
Sonoran answered you above.