Color can vary, there is a wide range of normal. That could be from berries/cherries. If they have access to wood ash, that can make droppings black or dark. Lots of things can change the color. If they are all acting normally, it's probably OK, but you can restrict their area and make sure the only thing they have is chicken feed, to see if it clears up, then you will know it's something they are eating. Also color can be deceiving depending on what it's on, if you take a bit and smear it on a white paper towel it may give a better idea of the actual color.