Black stool.... Taking folic acid supplement from 2 years is it due to folic acid?

Black stool. Black stool, possibly known as melena can be indicative of gastrointestinal blood loss and may be serious. Please see your doctor! folate (folic acid) shouldn't cause this but iron supplements can.
Black Stool. Generally, truly black stools are due to an upper GI bleed in your stomach or early small intestine. If you take iron your stools may look black but are really just dark dark green. Medically, being evaluated for GI bleed seems to be warranted. I doubt Folic Acid is involved in your case.