In my world I find it simplest to have blood iron based, the problem is that iron based blood is red and I want it to be black. Two things come to mind, one is making the blood denser, resulting in black blood and fast bleeders. The other is making blood richer in iron which leads to black blood but the threat of heavy metal poisoning. I am not a expert so I don't know if those would even work. What can I do to make iron based blood black and would the effect have on their body?