Welcome to BYC!
Blue legs?! That sounds horrible. How old are the girls, breed, and if possible pictures of them and the legs to help see the problem. Will be very helpful. The type of feed, are they free range? Are they still drinking eating on their own? If not do a search on the thread "go team tube feeding", they need to stay hydrated too.
I would give them a good going over to check them out under the feathers above the legs. Check pulse, see if blood return to legs by slightly pinching the leg and releasing to check for capillary blood return to area. Check the area they were in to see what they may have eaten, somethings thrown into a compost pile when decaying can make animals sick. Would smell bad rotten. Wild mushrooms are poisonous. Water that has been sitting and has some type of contaminate (fertilizer, bug spray) in it or poisoning from lead leaching into water. Plants, flowers in the area have they been nibbling on them? Some landscaping plants are poisonous also some weeds.
A friend has a Father who is an avid fisherman, he had redone his reel and left some fishing line in the area where her chickens were allowed to forage during the day. Some of her hens got small pieces of the line tangled around the leg just above scales and under feathers. The legs turned blue, and the line cut into the skin from tightening down. Other girls actually ate the line developed sour crop, impacted crop, and plugged digestive tracts. Two of those died as unable to clear the line from digestive tract. Surgery on crop of 2 others cleared out line and they are fine now. The hens with the line wrapped around legs also survived when imbedded line was cut out . The lengths of line left on the ground were less than a foot long. Area was raked to find all the remaining line. All of this happened within hours, was pretty severe in 24hrs. I am not saying this may be the cause, but chickens are pretty good about eating anything that looks appealing to them.
