It can be serious. Dark stools can be a sign that you are bleeding somewhere in your gastrointestinal tract (GI system). The stool is black (melena) because the blood has been digested. It can indicate that you have an ulcer or lesion in the tract somewhere. You need to be seen by your PCP or a gastroenterologist. A basic stool test can help.
Guiac. I would recommend you get your stool checked for blood. While I agree it can vary with your food intake, there is no sense in taking any chances. A heme occult stool test is easy, cheap, and quick. It will answer your concerns for blood in the stool. Best of luck.
Get checked. Stool color depends mostly upon what you eat. Dark brown stool is usually normal. If there is black in your stool then this can be a sign of bleeding in the upper gastrointestinal tract or it could be normal. If you have black or red in your stools then it is a good idea to see your doctor and get it checked out.
No but stay alert. the color of stool reflect what you eat so most of the time nothing to worry about. rarely in older population could represent a slow bleeding in you case again nothing to worry about.