Had a very dark, almost blackishstool this morning, after eating cereal with blueberries last night, is this normal or concerning?

NOT good if melena: Dark-colored stools may result from iron supplements, foods you eat (like a lot of spinach), substances (like peptobismol), but most significantly suggests bleeding. When blood is denatured by acid in your stomach, as from ulcers, stools will turn tarry ("melena"). Blood may also be ingested from nose bleeds, bleeding gums. Stop any nsaid's you're taking & call your doctor right away 4 evaluation.