I've been feeling more tired and have some black stools. When should I see the doctor?

Get eval now. This is suspicious of a GI bleed that is causing symptomatic anemia. Really need to get tested for anemia and get tested for GI bleed before considering benign options.
ASAP. Black stools can be a sign of internal bleeding, and the fatigue may be a symptom of anemia resulting from the blood loss. This is something that should be evaluated right away.