The short answer is: It depends. Sometimes black baby poop is perfectly normal, and sometimes it can be a cause for concern. The answer depends on the type of milk or formula your baby is drinking, how old they are, its consistency, and other factors.
When you first bring your baby home, their first few diaper changes typically feature a sticky, black poop-like substance that may resemble tar. This is called meconium, something that gradually filled your baby's system while you were pregnant, and the passing of it is not only normal, it's a sign that your baby's bowels are functioning properly.
Babies who drink iron-fortified formula may have black poop regularly in their diapers, and this is totally normal, too. Iron ingredients in their formula causes poop to turn a dark black color.
Cases, where dark black poop is not normal, is if your baby does not drink iron-fortified formula. Thick, black stools in 3-month-old babies (or older) can be an indication of bleeding in the digestive tract, and you should call your pediatrician right away.
Dark grey baby poop
Baby poop that's dark grey is a cause for concern. If you see dark grey baby poop in your child's diaper, you should contact your doctor as soon as possible. It could mean they're not digesting food as they should and getting all the nutrients they need. The same goes for poop that looks chalky and white.
As mentioned above, black baby poop can be caused by drinking an iron-fortified formula. Many babies drink this type of formula to ensure they're getting enough iron in their diets, and it can turn their stool a black color. This is not normally a cause for concern, but it's always a good idea to check with your pediatrician to be sure.
Sometimes, you may notice flecks of blood in a baby's poop that looks black, kind of like poppy seeds, which is an indication it's been digested. Typically this comes from breastfeeding and baby swallowing bits of blood from mom's nipples that may be swollen or cracked. This is also totally normal.
As your child grows older and expands their diet to include solid foods, black poop may still appear from time to time, but this is usually due to ingesting certain foods. For instance, black licorice, grape juice, blueberries, or even Oreo cookies can turn a child's stool partially black.
Medicines can also be a cause of black poop, most notably antacid medications usually taken to treat upset stomach or heartburn. Iron supplements, whether taken as multivitamins or as part of iron-fortified formula, can also cause stools to turn black.
Black baby poop that is thick, dark, and tarry — almost like the meconium but a bit firmer — may suggest a serious problem, and you should call your pediatrician right away if you notice this.
They can likely perform further tests to determine whether or not the black baby poop is caused by bleeding in the digestive tract or something else.
Your baby's body tends to absorb breast milk more completely, so it's entirely possible they may not poop for days while being breastfed, and this is normal. But other breastfed children may pass a yellow-colored stool that's mustardy and seedy in consistency after each feeding, at least for a few weeks or months. Formula-fed babies' stools tend to be darker or tan-colored.
Black baby poop that looks black but isn't
Sometimes baby poop that has bile in it can look black in dim lighting or indirect light, but it's actually very dark green. If you put a bit of it on a cotton swab, smear it on a piece of white paper, and hold it under a bright light, it will look dark green instead of black.
This is normally nothing to worry about, since sometimes digestive juices from the stomach or intestines can come out in baby's poop. If you're worried, you can always call your pediatrician.
Whatever your baby eats or drinks is going to affect what ends up in their diaper. The milk your baby swallows, whether breast milk or formula, goes into their stomach, where acids break it down. Nutrients and water get absorbed into the blood to keep your baby healthy and strong, and the other indigestible things, like fiber, keep moving through until eventually it comes out as poop in your baby's diaper.
If it moves through your baby's system at a slower pace, it can pick up water and other nutrients from it, and the result is a firmer poop. But if stuff moves through too quickly, water and electrolytes don't get absorbed, and sometimes this results in diarrhea.
So while some colors are entirely normal, others are a cause for concern. If your baby's drinking iron-fortified formula and you notice black poop baby is passing, then it's probably nothing to worry about. But if you notice it out of the blue and they're not taking iron supplements, then you may want to call your doctor to rule out something more serious.
To sum up, black baby poop is normal in some cases and not normal in others. The answer in your personal situation depends on your child, their eating habits, and other symptoms. Remember that your pediatrician can always help answer questions if you're unsure.