I'm so sad she couldn't have pulled through. You tried harder to save her than most people.
The black in the poop may be from internal bleeding. It adds more mystery to this. If you are close to a state lab that does farm animal testing, you could take her to them and have them do a necropsy. It would answer all the questions. You need to refrigerate her right now while you think this through. She would need to be tested withing 24 hours, 48 the limit, but she needs to be kept cold.
You can opt for testing that poop, but it might cost as much as a necropsy would, and wouldn't tell you more than if she had worms or coccidia and if it contains abnormally high levels of bacteria and what they are. (called a gram stain test) If you collect that poop in a clean jar or new plastic baggie, refrigerate it and get it to your vet in the morning for a gram stain test and fecal float.