My 3 year old has just had a dark greeny/black poo, he seems fine but what could this be?

Could be iron?: Is you child taking iron syrup, or any iron medication? If he is, that would explain the stool color. If your child is not taking any iron, it still may be due to foods he ate, such as spinach, avocado, etc. If he has only one stool of the dark color, and has no other symptoms, i would not worry. Blood from upper intestinal tract would produce black stool, and this needs a doctor's attention.