Fun moment in a game of mine.

So I have a doctor of biology in one of my d&d games and in our last session I was describing the black blood of a certain monster and one of my players goes:
Player: "Is it actually possible to have non-red blood?"
Doctor: "Well horse shoe crabs actually have Cyanin instead of Hemoglobin in their blood that has copper instead of iron in it and that makes their blood a really neat blue color."
P: "If it's the type of metal in the cell that determines the color of the blood, what kind of metal could make blood black?"
D: "Well it's actually how the metal reacts to oxidization that determines the color. Copper turns blue from... uh..."
Me, being helpful: "Patina!"
D: "Right, and iron goes red like rust."
P: "So what could bond with blood and then react black?"

Cue a 10 minute search through the periodic table and different oxidization reactions.
Turns out, Maganese oxidizes black and is right next to Iron on the table! It could theoretically bond to blood cells and if it did, would make black blood. Do with this information as you will.