In the Pern books (McCaffery, Anne), dragons and other beings have black blood due to Boron. Not sure of the science behind it. Fans of her books most likely will attribute using Boron as the reason as copying. If there are scientific reasons, thanthen use it.
.:.
I
On the other hand, I have Hemochromotosishemochromotosis which results in high levels of iron. I haven't seen any change in colour when my iron levels are too high. The treatment is withdrawing blood so extra is made to dilute my iron rich life blood. From this experience, I advise NOT explaining black blood being due to excessive iron.
.:.
.:.