Is this something to be concerned about?
This could be related to blood in the stool – usually degraded blood which may have originated from the upper gastrointestinal tract. Usually it makes the stool black, tarry and has a somewhat foul odor (more than the usual for stool). This is known as melena. Yes, it could be serious and you should see a doctor.
Of course, this may also be related to constipation, partial bowel obstruction and certain foods that you may be eating. A colonoscopy and endoscopy may be advisable so your doctor will refer you to a gastroenterologist.