Hi i am Dr.kadhir,
At first it is important to know whether it is dark green or grayish black. Dark green color are usually misinterpreted. Blood can also cause the black color, various food and medicines can give similar appearance.
It is important to take a focused history and examine the child . If the child is active, taking feeds well you can wait and watch for few more hours. If she continues to pass black stools it is important to confirm it as blood.
If she looks sick then better to obtain a pediatrician
Thank you for your consultation with health care magic.