If you just want black blood because it would be cool ( and it would) have the blood become denser due to a much higher need for coagulation to stop bleeding.

You could also have a "black" blood cells that replaced the white blood cells in terms of what the white cells ( more info here: https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=160&ContentID=35) do. It could be do to a disease factor as We (and i'm using the royal we) have cured all the diseases out there so evolution tried a survival of the fittest out and gave us a new disease (think black death) and our blood evolved to have stronger "white" blood cells that turned darker, causing the red iron blood to turn black. our scientists are not sure why yet but are working very hard find out.

Due to that fact, doctor's can now tell immune compromised children by how dark the blood is.

If you just want black blood because it would be cool ( and it would) have the blood become denser due to a much higher need for coagulation to stop bleeding.

You could also have a "black" blood cells that replaced the white blood cells in terms of what the white cells ( more info here: https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=160&ContentID=35) do. It could be do to a disease factor as We (and I'm using the royal we) have cured all the diseases out there so evolution tried a survival of the fittest out and gave us a new disease (think Black Death) and our blood evolved to have stronger "white" blood cells that turned darker, causing the red iron blood to turn black. our scientists are not sure why yet but are working very hard find out.

Due to that fact, doctors can now tell immune-compromised children by how dark the blood is.