Naturally, there’s more than just one reason for black stools.
Are you taking an iron supplement? Or is iron in a multi-vitamin you’re perhaps taking? Unabsorbed iron salts cause a black stool. Temporary black stools can mean that you have blood in your stool.
Dark or black stools often refer to mucus-type plaque, which is an accumulation of toxins in the intestinal mucus which are then expelled. As it passes through the colon, the toxins are re-assimilated back into the body; this can also explain the presence of fatigue. This mucus plaque is not die-off but it’s a cleaning effect which is normally a good sign.