Please take no offense, but I will propose a couple of possibilities. First, SS, like
Tractor Supply, merely re-sell hatchery chicks. Hatchery chicks are mass produced and the breeding isn't as important as the sheer numbers that can be sold at a profit. Thus, some chicks said to be this or that "breed" are very poor examples. Doesn't make them any less a chicken and indeed, they may be healthy, happy and great birds, but they are far, far from type or standard.
Secondly, feed stores are notorious for mixing up chicks. The staff has a lot to do and they aren't highly paid positions. Customers too pick up chicks and put them back into wrong bins. Between the hatchery, staff and other customers, all kinds of whacko things happen.
So, if the chick is healthy and growing, that's about it. Find the positive and move on. You might find the black chick to be an Australorp or Black Sex Link pullet and you'll be fine with it, as time goes on. It is also quite possible that it is indeed a poorly feathered Barred Rock, just doesn't have the correct barring.
Best regards,