In about 30 years of various attempts and five dogs, the closest I ever got to pure black was my first, a Black Lab/ Irish Setter cross who looked like a jet black Golden Retriever. Unfortunately, he had about twelve white hairs on his chest. All the others have been black and white. A jet black Great Dane would be really cool.
Are you sure he was a mutt, and not a Flat-Coated Retriever?