If your taking prenatal vitamins, there is iron in those too. That could be it but I would double check with your OB
ask your dr....diarrhea can be that color when it's caused by hormone fluctuations, but the black color can also indicate old blood in your digestive system. call OB and ask...may ask for stool sample, but they'll probably tell you it's normal ...i THINK i remember something like that my first trimester, and the iron in the pre-natals may be showing up in your morning stools.