There is a difference between "dark" and "black." Normal poop color can range from very light brown (like cardboard) to very dark. Black stools caused by digested blood from the stomach or small intestine are BLACK black, and sticky. You will look like you passed some tar.
I understand how you feel, as I've had a couple of bouts of very dark--but not tarry--bits of stool in the past week. It coincides with taking a new multivitamin that contains 100% of the recommended iron dosage, so I'm thinking that's what caused it. I'm off the vitamin, so if it happens again this week, I suppose I'll deal with it then. But don't jump to blood being an immediate cause. And even if it were, while it warrants investigation, there's virtually no chance you'd bleed out from it.
As someone else said, you can get an at-home test from many pharmacies. It's just some paper you drop in the water that turns blue or green if there's blood in your stool. That may put your mind at ease until you see a physician. Good luck to you.