My sick Cornish x is blue?

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We are raising Cornish X for the first time. They are in an outdoor chicken tractor and have been doing well. We found one today that is lethargic and her skin is very blue. Her breathing doesn't seem to be labored or raspy....any ideas what it is? We have isolated in her own pen and she is in fresh shavings and warm. But she isn't eating, drinking or going poo. The chicken is 6 weeks old.