what does it mean if you are having this dark greenish almost black looking bowel movement?

Are you taking iron supplements? That could do it... Have you taken any pepto bismol lately? That could also do it. How is your diet? How long has this been going on and how old are you? I mean if it's been months and you are let's say over 60 it could be something serious. If it's been going on for a few months, no matter how old you are, you should mention it to the doctor.