I've had both Black and Grey. I think the Greys are more mainstream and the blacks should have a more chickenly flavor.
I spoke with a lot of customers without giving away they had eaten two different breeds two weeks apart. I asked "Where these ones (which were Blacks) as good as the last ones (which were greys)?" Universally everyone said they were just as good. So no one tipped a preference in flavor of one over the other.
Could I tell a difference? Maybe. It's really hard to say knowing I had prior knowledge the blacks should taste more 'gamey' or rich. All the farm chicken I've produced tastes great to me.
Regarding meat toughness... this really has little to do with the breed or chicken. It's a reflection of the living conditions. The more the bird exercises, the more muscle texture will form and you will get 'tougher' meat. However, they actually taste of chicken when you let them roam. The garbage at Safeway is flavorless and is full of water. Our problem as chicken people is that's what people think chicken should taste of. So we can go too wild without the risk of alienating our customers.
Also, the flavor and texture of the bird can be influenced by their diet and what grains they are getting. So it's hard to say that one breed will be tougher, stringier, moister or tenderer than another.... except you can say, without exception, that heritage breeds will be narrower and thinner breasted and the meat is always a bit tougher. Crossbreeds simply are the way to go with your meat chickens.
p.s. We do raise pork. It's one of the animals I may be able to go organic with eventually... after our pastures and field crops get certified. The others are duck, goose and maybe meat chickens. But, as far as goats, sheep and beef go, we're not there yet. Given the ammount we deworm (based on fecal exams and anemia checks) to suddenly stop deworming I suspect we'd lose a vast number of animals. Organic can be a bit cruel in my climate.