Welcome to HealthcareMagic.Presence of black stool generally is a marker of bleeding taking place somewhere in gastrointestinal tract, mostly in upper part. Stool for occult blood is indeed a test which shows, presence of that bleeding, but at times first test might be falsely negative. So pl get it repeated.Also you need to correlate with complete blood count
. If the haemoglobin
and other related parameters are also abnormal.If you are on iron supplements
or beet may also lead to black stool.You can take a second opinion
from another gastroenterologist.May undergo a antihelminth course(deworming)too. Take care.