So, the general idea is this:
A humanoid species' blood uses a chemical to carry oxygen throughout their system. When iron is introduced into their bloodstream by say, stabbing them, it results in the chemical reacting with it and creating an acidic substance, causing pain and thus creating the (in a way true) legend that these creatures are vulnerable to iron.
Now, this is for a fantasy setting, so the material doesn't have to be a real chemical. I just want to know if this idea seems possible. Could be that the iron reacts with the compound containing the chemical, which then results in an acidic residue due to the leftover elements.
As for blood coloration, I'm thinking the chemical would give it an orange-ish or peach-like color when in the body, but then take a golden, metallic hue when clotting outside it. A bit out there but still.
Update: Pyrite! Apparently Fool's gold is more dangerous than it seems.
Pyrite Disease is caused by the mineral being exposed to humid air, breaking it down to iron sulfide, corrosive sulfuric acid and sulfur dioxide gas when it reacts with oxygen and water! Now, this isn't exactly what I want, but a good base to work off of.
Let's say there's a mineral similar to pyrite but with a different composition (no need to go into extreme detail). It still has that golden color, but is able to be incorporated into structures similar to hemoglobin in the human blood. Additionally, due to its exotic composition, it only produces those harmful chemicals when exposed to two things in excess: Iron, and Oxygen.
Pyrite + Oxygen + Water= Iron Sulfide, Sulfuric Acid and Sulfur Dioxide Gas
Unique Metal + Oxygen + Iron = Iron Sulfide and Sulfuric Acid. Certainly not a reaction you'd want to have in your veins.
Sure, someone trying to tie it down with precise chemistry may need to make some acrobatics with its formula, but at the end of the day the result is ideal: A replacement for blood that has a metallic gold color, and results into a corrosive substance when in contact with iron.
I'd love to hear some feedback for this idea!