I take Dexilant which is a version of lansoprazole and I have never had black stools from it. You can get black stools from licorice and some other types of foods with coloring as well as Pepto Bismal. I have had bright green stools from black licorice as well, that was weird but normal apparently. Red jello turned my stools almost blood colored once and had me worried.
For sure follow up on it with a doc if you have no source for the coloring from your food.