Upper Gastrointestinal Bleed (UGIB): This is the most likely diagnosis due to the presence of black tarry stools (melena), which is a classic sign of upper GI bleeding. The accompanying jaundice could be due to a significant bleed leading to hemolysis and subsequent bilirubin elevation.

Hemobilia: This condition, characterized by bleeding into the biliary tree, can cause both melena and jaundice. It's less common than UGIB but should be considered, especially if there's a history of liver or biliary disease.
Hematemesis with Melena: While hematemesis (vomiting blood) is more obvious, some patients may only present with melena if the bleeding is slow or if the blood is fully digested. Jaundice could be secondary to liver dysfunction or a large bleed.

Malignancy (e.g., Gastric or Esophageal Cancer): Although less common, malignancies can cause UGIB and should not be missed due to their significant implications on prognosis and treatment. Jaundice could be due to metastasis to the liver or bile duct obstruction.
Portal Hypertension: This condition can lead to varices, which are prone to bleeding, presenting as melena. Jaundice may occur due to underlying liver disease (e.g., cirrhosis).
Vascular Malformations or AVMs: These can cause recurrent or severe bleeding and may present with melena and jaundice if the bleeding is significant.

Dieulafoy's Lesion: A rare cause of UGIB, characterized by a small, usually solitary mucosal defect with an exposed, often large caliber, artery at its base. It can cause significant bleeding and melena.
Gastrointestinal Lymphoma: Although rare, lymphoma can cause UGIB and present with melena and jaundice, especially if there's significant involvement of the GI tract or liver.
Ingested Foreign Body: In rare cases, ingestion of a foreign body can cause UGIB, especially if the object is sharp or causes erosion into a blood vessel. Jaundice would be less common but could occur with significant bleeding or if the object obstructs the biliary tree.