Jennifer Shu, MD, FAAP, Pediatrician The first few stools of a baby's life will be blackish-green and ... A watery stool that occurs more frequently than usual can be a sign of trouble.

meconium is a sign the baby's bowels work. It usually transitions to greenish-yellow poop by three or four days. Babies who drink iron-enriched formula may have very dark green or black diarrhea.