I agree with Sally, you probably want to worm your geese. I have 4 Sebastopols and although they are much more hardy than my chickens, I do worm them.

I just had a chicken at the vet for boarding (lol, don't even ask) and my vet mentioned that her poop was like tar the first day. Turns out she had an infection in her leg and he put her on Sufatrim immediately. It is antibacterial and anti-viral, so it is his go-to when anyone is sick. I wonder if the black poop is a sign of infection?? He is an avian vet and was very concerned when he first discovered it. Just a thought. It won't hurt her to get her on some antibacterials. And from my experience, they go downhill so fast...If she is losing weight and already showing signs, I would get on it RIGHT AWAY.

Good luck! M