Coat color can be deceiving. The golden coat that Golden Retrievers are named for is caused by a mutation in one gene, called the E locus. This trait is more dominant than other coat colors. When a dog has this genetic color, they’re not able to express any black pigment, even if other genes (such as the K locus) indicate that the dog should be black.
Because two Golden Retrievers will always produce gold puppies, most people don’t realize that Goldens still carry the “instructions” to produce other colors, like black and brindle. When you cross a Golden with another breed, the other colors may be expressed in the puppies. Frequently, this will be a black coat.