Thank you for your query.
You mentioned that your daughter is passing dark black hard stools. This indicates that she may be constipated. It is suggested that you increase her fluid intake which shall help in the constipation
and also increase the frequency of urination.
As the appetite is good I think you need not worry. Such changes do happen at times because of change in environmental temperature or the outdoor activities of a child.
I hope I have answered your query to your satisfaction.