Hi welcome to Health care magic..........
Black coloured BM suggests blood in the stool and needs evaluation if it persists.
Consult a gastroenterologist
Get a stool for occult blood test
done and if positive menas there is blood in the stool and needs evealuation.
Casues can be 1) A bleed due to meckels 2) A av malformation in the stomach 3) worm infestation etc.