I would seriously question the breeder. Who sells animals with such a problem?! This may be a notifiable disease.
I don't know what it's like where you are, but here's some theories/suggestions:
Frostbite? (Long shot but I don't know so thought I'd ask).
Were they in a muddy pen, or one that stank of ammonia? (Could be ammonia burns or similar).
Feet caked in something? (Have you tried washing them? Even if they were caked in acidic poop, they could have developed sores, which could have become necrotic due to lack of sanitation... Scabs can be black).
Lack of bloodflow? (Some diets cause cardiovascular problems, particularly males being fed layer feed, and clots or loss of circulation are par for the course in that case. Gangrene follows loss of oxygen when it's prolonged).
Are the feet hot or cold? (This will tell you if they're getting black feet due to loss of circulation, or if there's an infection or fever, but not much else).
There is a chance this is a serious infectious disease, like a pox. I would try to ascertain for sure ASAP.
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Yes, it's a behavior that shows sore feet, although normal birds may adopt it occasionally. Not a whole flock though. If they look normal now but are standing on one foot I would inspect them carefully. This makes it sound even more like an infectious disease, to me.
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If they're not used to pine shavings they may eat them and die. If possible I'd use something else in that case.
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Can you elaborate on why you put them down? The symptoms may be relevant.
Best wishes.