Hello! I had gone through your query and understand your concern. Black tar stool in medical term is called Melena and is a concerning finding because it indicates gastrointestinal bleeding probably by the stomach. Some foods and medication can confuse because they give a darker color to the stools but not tar black color. You should be worry if the stool color is black black like tar,not only darker than usually. I suggest to consult a pediatrician as soon as possible to establish the diagnosis. A blood stool test should be done to confirm the presence of blood in stools. Then other studies can be done to find the right cause. Hope I was able to help you. Wish good health to your child. Dr.Rovena