Well as you describe,as it's single first episode of black spots in stools then you shouldn't worry and just consider wait and watch approach.
However such black spots in stools can happen due to eating certain foods, taking iron supplements, or possibly from internal bleeding.
Black licorice, blueberries, Lead, Bismuth (Pepto-bismol) can also cause black stools.
Bleeding can occur due to bleeding ulcer, gastritis, esophageal varices or a tear in the esophagus.
You need to get your self examined by your doctor if your symptoms are recurrent and get stool examination done to confirm the presence of blood.
If no bleeding is there then treatment is to make some changes in your lifestyle and diet that are caused of this black bowel movement.
Otherwise you shouldn't worry and eat fibre rich diet.
Hydrate yourself with plenty of fluids.
And watch for repeat such unusual symptoms.
Hope this helps.