My three year old is having black stools for the past two day's what causes it?

BLACK: stools are concerning since they can represent upper (stomach) GI bleeding. HOWEVER, dietary intake ( eg: blueberries) or IRON supplements can also result in blackish stools...A SIMPLE STOOL TEST will give the answer...get it ASAP... Hope this is helpful! Dr Z

Diet: It is likely from the diet. If he appears unhealthy and this issue persists for more than a week, talk to your doctor.