Photo by Djanick Michaud.
Most golden retrievers don’t have any black hairs on them, but sometimes, they experience somatic mutations that allow black pigment to appear on their fur.
Djanick Michaud sent me this photo of this dog with just a few black hairs that are likely the result of one of these mutations.
It’s not heritable, but you can get some weird black hairs in a purebred golden retriever.
This can happen in yellow Labradors and buff/red Cockers as well. Strangest one I’ve seen was a Golden with multiple black splotches like a harlequin Dane, but black on golden. Accented by a white face blaze and white “shirtfront”, he caused a lot of double-takes. More often the black is a single spot, anything from a few black hairs to perhaps the size of a dinner plate.