The more information you give, the more and better help can be offered-
All I can get from your post is the breed and she has very severe diarrhea.
How old, where did she come from, how long have you had her, what are you feeding them, do you deworm, do you treat for mites/lice, are the others ok, have you inspected her besides noticing her messy bottom? Seems to be eating? Is she eating or not? Pick her up- feel her crop, feel her keel, clean off her bottom and look at it and around it- let us know what you find and more about her history.
Black feces is possibly GI bleeding, never a good thing. Dark green feces means she is not eating. White urates are normal, yellow is not. It sounds like you have a very ill bird, possibly dying. If you cannot take her and a fecal sample to a vet- you need to up your supportive care. Besides quarantining her- offer a heat lamp, offer tasty and nutritious food, consider adding electrolytes to her water (bad diarrhea mean loss of water and electrolytes, add that to not eating & drinking well- this can kill a creature just as fast as whatever is causing the diarrhea). If she is not eating/drinking- if you are comfortable slowly syringe feeding her, this may help her.
Long story short, best chance this bird has is having a hard diagnosis- ie fecal check for severe parasite problem, bloodwork to see if she has organ failure or severe infection, and a good physical. This can help you decide if you have a bird that has a chance of recovery, or whether your should cull her.
If you can't do this, then best thing is to do supportive care and hope.