Usually if there is one bully pecking feathers, only the bully needs them. Since feather picking can become a bad habit if more are doing it, then you may need to put some on others. The main thing is to address the cause of the feather picking.
I also recommend human super b complex, 1/2 tablet daily given orally or crushed into a small treat of egg. Walmart sells Equate brand for about $8. She could have injured it causing some nerve damage. Have you added any new birds in the last weeks or months who could have been a carrier for Mareks?
How cold has it been there? Can you get a picture of the comb after it gets normal again? Usually a dark comb is from poor oxygenation or heart failure, but she might be getting some frostbite. Can you make sure that she is drinking enough, since dehydration could also be a problem.
Use some electrolytes and dip her beak as often as you can. Poultry NutriDrench 1 ml daily could also be used as a rescue tonic. It could be shipping stress or a weak chick, but I would try hard to save it. Is it pooping, or have any pasty butt/stuck poop on vent? Is it under a heat source?
How old is she and has she laid before? You could try giving her a human calcium citrate with vitamin d tablet 300-600 mg orally, or a Tums broke in two orally. Place her on a warm towel to sit.
The tablets are 10 mg and most sources say to treat at 10-15 mg per kilogram (each 2.2 pounds) twice a day for 5 days. I would weigh her to make sure her weight is 3 pounds.
The feet look very irritated and the ankle joints look enlarged. Is there any chance of frostbite? There may be some scaly leg mites and possible mycoplasma synovitis (MS) instead of gout. Gout causes large lumpy toes and feet, while MS causes swollen ankle and hock joints. If the temps are not...
Is everyone still acting normally, eating and drinking? I would ask your vet if they will do a fecal float on some fresh droppings collected in a ziplock bag. If they won’t without seeing the chicken, I would go ahead and worm them. Worms can cause blood in the droppings.
Wry neck (torticolis) is a common neurological symptom when there is a neck or head injury. Separate him so he doesn’t get picked on close to food and water, and feed him both every 2 hours. Sugar water 1 tsp per cup or electrolytes would be good while he is in shock. I would get some human...