Poo is dark because of blood cells being excreted, the darker it is the more blood in the poo. Thats if its being released higher up in your intestinal tract which would be a more ominous sign. If its being released close to the anus it is bright red and visual and most likely an internal hemmorhoid. But no one way to tell without appropriate medical advice. Colonscopy, why not, at least you know that nothing is growing in there. They also can do poo samples, just do some swatches for the doc, kinda like picking out carpeting.