A black "Golden Retriever" is actually called a Flat-Coated Retriever. This breed was developed from two American breeds, the Labrador Retriever and the Newfoundland. Therefore, it is not considered a purebred Golden Retriever.

The black "Golden Retriever", or Flat-Coated Retriever, shares some visual similarities with Golden Retrievers. Golden Retrievers come in approximately nine different colors, ranging from off-white and cream to dark gold. However, black is an extremely rare color in Golden Retrievers and is generally considered a result of a genetic throwback.
While Flat-Coated Retrievers may bear a resemblance to Golden Retrievers, it's important to note that they are distinct breeds. Although both are gundogs, the Flat-Coated Retriever is not a Golden Retriever. The Flat-Coated Retriever was bred from the Labrador Retriever and the Newfoundland. The history of the Flat-Coated Retriever dates back to the early 19th century, much earlier than the Golden Retriever, which only has a little over a century of history. Flat-Coated Retrievers were originally used for hunting and guarding, but they declined in popularity until around the mid-1960s, when breeding programs were gradually re-established.
Flat-Coated Retrievers have a more streamlined build and more defined musculature than Golden Retrievers, and they also tend to project a more rugged appearance. They are considered excellent hunting dogs, known for their agility and precision.
Is a Black Golden Retriever Purebred?
A black "Golden Retriever" is typically not a purebred Golden Retriever. If a Golden Retriever puppy is born black, it suggests that the parent dogs may not have been purebred or could have had mixed breeding or a genetic mutation. Purebred Golden Retrievers are typically characterized by their golden fur, a tail that curls upward, a moderately long muzzle, and a prominent black nose. It is also possible that a black "Golden Retriever" is not actually a Golden Retriever, but rather a Flat-Coated Retriever.