Hello and welcome to healthcare forum.
Black coloured stool is known as melena, which suggest that there is bleeding some where in upper gastrointestinal tract.
First of all go for routine and microscopic examination of stool to know weather the black colour of stool is due to blood or not.
Then consult a gastroenterologist
who will advice you to undergo upper g. i. scopic examination to know the cause of bleeding.
I hope i have answered your query.