One probable cause for a dark colored vomit may be that your child has developed stress gastric ulcers
. The child will need a thorough examination and a battery of blood tests. Kindly don't ignore this complaint and see a pediatrician
as soon as possible. If the child has stress ulcers she will be relieved with antacids and other drugs. She may have a bleeding disorder
too. Depending on your location she might dengue hemorrhegic fever too. She definitely needs visit to the doctor.