To my knowledge of oxazepam, there should be nothing in the pharmacology of this medication to cause a change in stool color.
Black stools are usually the result of digested blood or substances such as:
If you have black stools that you can not attribute to a food or to iron supplements, see your doctor as soon as possible. If the black color is accompanied by other symptoms such as vomiting or acute pain, seek medical attention immediately.
Additional information: Bloody or tarry stools: MedlinePlus Medical Encyclopedia
Black stools are usually the result of digested blood or substances such as:
Black Bleeding in the upper gastrointestinal tract, such as the stomach. Iron supplements, bismuth subsalicylate (Kaopectate, Pepto-Bismol), black licorice....Source: Stool color: When to worry - MayoClinic.com
If you have black stools that you can not attribute to a food or to iron supplements, see your doctor as soon as possible. If the black color is accompanied by other symptoms such as vomiting or acute pain, seek medical attention immediately.
Additional information: Bloody or tarry stools: MedlinePlus Medical Encyclopedia