I have black stool why did doctor prescribed prilosec?

Stomach issue.: Your doctor may think you have gastritis or an ulcer. Black stool implies that there's an upper tract source of bleeding into the intestinal tract. If it doesn't stop you may need further testing.

Bleeding ulcer: Black stool can be due to blood from as high in the GI tract as your stomach. As red blood makes its way down your GI tract it can turn your stool black and therefore the rationale for prilosec is to reduce the acid which may cause some gastric bleeding. (It may be a stretch without a true diagnosis via endoscopy first).