Owning Koi myself, I will tell you that if you do take these koi, anyone serious about their pond/fish wouldn't buy them.
KHV runs high now a days, and will wipe out your stock.
Also, from living in a natural-esque pond, they would more than likely be ridden with parasites and bacteria- and once put in a home pond would more than likely stress out from such different water parameters, and treatments needed.
Koi are more sensitive to sudden changes than people think, and you would spend more time dealing with dead koi than making $$.
And more than likely the "black" koi are typical carp. Black koi are hard to come buy unless the original buyer bought them from someone who imported koi from Israel- and they have actually been noted to carry KHV more than any other origin of Koi.