Its good to eat healthy and excercise to be fit.Foods which can lead to dark coloured stools are beetroot, somtimes red meats ans also the various types of seasoning done in food.
Besides eating healthy are you on any kind of protien , multi vitamain or iron supplements
? supplements such as iron are also known for dark coloured stools.
If none of the above causes it could be pathological.Black stools with blood is called malena. There could be number of reasons such as gastritis
, interstinal bleed etc. The best way to find out is a simple stool which will atleast rule out blood in stools.