While people claim a strict pairing of purebred Golden Retrievers can produce black puppies, breeders—and the science of genetics—don't seem to agree.
A few breeders say black puppies could be the result of genes from a distant ancestor sneaking through. The claim is that a stray dominant gene may cause the Golden's black-pigment-blocking genes to turn off, thus allowing the color to present. Because part of the Golden Retriever's genetics is the gene that blocks black pigment in order to create the golden color, this theory is widely disputed.
Many others insist a black Golden Retriever must be the result of an unintended breeding or even a case of mistaken identity—perhaps the dog bred was a Flat-Coated Retriever instead. Any way you shake it, a black dog—no matter how much it looked like a Golden Retriever—could not be registered as such by the AKC.
There is a gene mutation that may cause black splotches to appear on a Golden Retriever. This somatic mutation is not genetic and won't pass to future generations.