Thanks for your query.
We need to look into the following issues:
1. Arnold-Chiari malformation
2. Black stools
Black stools are not related to Arnold-Chiari malformation. Black stools can depend on the food taken earlier or passage of blood. Stool examination for occult blood will reveal if it is due to altered blood.
You mention that the tremors are not related to the malformation and that she could not hold her drink. It is better to get a fresh neurological examination done to find the cause of the tremor and to know whether the malformation is causing more symptoms. In case the malformation is causing problems then surgical resolution shall have to be planned.
I hope I have answered your query to your satisfaction. Please feel free to revert back in case any clarification is required or you would like to share more information.
Wishing your daughter all the best...