I am so very sorry for your loss.
You will never know, and that will be difficult, but you will honor your dog by learning and always remembering.
The black stools is very clearly an internal bleeding issue. You said she did not eat much at night, she may very well have had problems unrelated to the chicken bone. For example, GSDs are very susceptible to a cancer called hemangiosarcoma, and they have internal bleeding which may go unnoticed, until the end where they start bleeding out. This is what my GSD died of.
GSDs are also very prone infection as a breed, wo an intestinal infection is also suspect.
After no food, and maybe toxins affecting the body and brain from either infection or cancer or the body shutting down, that may mimic symptoms like you describe, as can lesions in the brain, but that wouldn't address the bleeding internally.
The chicken bone, you didn't mention if it was cooked or raw. Cooked bones can splinter, nearly impossible to say if it had anything to do with this, though maybe there was a bowel perforation.
It's a tragic loss, please be kind to yourself, she would not want you to hurt as you do. Remember her, and honor her by uing these lessons to better care for your next animals, that is about the bet that we can do.