Black Chicken Toes

[p1cklen2s]

I found my chickens had been pecking on the toe of another overnight. I separated the injured bird, and her toe has remained black. That was about a week and 1/2 ago. I have now discovered that all 3 of my other chickens of that same breed have developed the same black areas of their toes. It is only in this particular breed though. They were in a pen with 5 other chickens of another breed, and all 5 of those chickens' feet are healthy. I am attaching a picture, can anyone tell me what this might be and how to treat? I have searched online and haven't been able to find a description that fits the condition.