Swallowed Maternal Blood: This is the most common cause of tarry black stools in newborns, especially if the mother has cracked nipples or is bleeding during breastfeeding. The blood is swallowed by the infant and then appears in the stool.

Gastroesophageal Reflux Disease (GERD) with bleeding: Although less common, GERD can cause irritation and bleeding in the esophagus, leading to black stools.
Allergic colitis: An allergy to cow's milk protein can cause inflammation and bleeding in the colon, resulting in black stools.
Infectious colitis: Certain bacterial or viral infections can cause colitis, leading to bleeding and black stools.

Intussusception: A condition where a part of the intestine telescopes into another part, causing bowel obstruction and potentially life-threatening bleeding. Although less common in this age group, it's crucial to consider due to its severity.
Volvolus or Midgut Malrotation: These conditions involve twisting of the intestines, which can cut off blood supply and lead to tissue death and bleeding.
Hirschsprung's Disease with Enterocolitis: A condition characterized by the absence of nerve cells in parts of the colon, leading to severe constipation and potentially life-threatening enterocolitis.

Congenital vascular malformations: Abnormal blood vessel formations that can cause bleeding in the gastrointestinal tract.
Gastrointestinal duplication cysts: Rare congenital anomalies that can cause bleeding if they become ulcerated or rupture.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding due to the presence of ectopic gastric mucosa.