Get your stool tested for occult blood in any good lab. If negative no need to worry. If occult blood is positive then you have to consult a specialist gastroenterologist for opinion.

If you continue to have black stools without taking any iron supplements or vegetables such as beetroot then you need to consult a Gastroenterologist. You will need medication in the form of a proton pump inhibitor such as pantoprazole. You may also need an endoscopy to check the inside of your stomach as an ulcer could be the cause of black stools. There is no connection between the delay of your periods and passage of black stools. Any form of stress can delay the starting of your periods.