Thank you for your query,
The black color stools may be due to using oral iron suspension or due to a upper Gastrointestinal bleeding
If you are not giving any iron preparation, it may be due to some bleeding in upper Gastrointestinal bleeding.
So give stool sample for occult blood and for RBC
to detect any bleeding.
If the stool is normal nothing to worry and confirms there is no bleeding.
So consult your Pediatrician
for further advise.
Hope I have answered your query.