I'm having the same thing and I'm 7 weeks :/ hopefully everything is ok...
Black stool can be from vitamins, but it can also be concerning. Does it look like coffee grounds? If so this is something that you need to see a doctor for, it could be bleeding in the GI tract. I would definitelty get it checked out.
It's normal if your taking iron supplements or if there is a lot of iron in your prenatal vitamin