It is very rare bumping in a Black Golden Retriever, also the ones referred as such should be Flat Coater retriever. The standard colors admitted for the Golden are the ones that goeas from cream to golden, while there are two colors for the Flat Coater Retriever: black and liver. So, those that seems to be a Black Golden Retriever are likely to be Flat Coater Retrievers.

While the Black Golden Retriever does not exist for standards, since the golden color of the Golden is obtained through cross breeding perfectly calculated according to genetics, it is true that some American texts and articles confirm the existence of the Black Golden Retriever, which should be extremely rare.  

Black Golden Retriever

It is expected that it is a mere 0,4% chances to have a black puppy from a litter: it has been recently estimated thet only 1,500 of them are living in Usa and there are no specialized farms for Black Golden Retrievers.

Black Golden Retriever, genetic features

But how can a Black Golden Retriever come into being? According to some American files, the black coloring depends on a recessive gene that remotely shows up. This is due to the fact that during the selection for Golden Retriever breeding, people have used Flat Coated Retrievers with dark fur, black or liver. The size at wither could overlap between Golden Retriever and Flat Coated Retriever, even if the last one is slightly taller and thinner. The male Golden Retriever has a height at the wither between 56-61 cm (weight 25-32 kg).

Even in Italy the Flat Coated retriever or the Black Golden retriever are not common, but obviously the price depends on the bloodline, the pedigree and the type of farm. They are also very rare, expensive and demanded in Usa.