I think it could actually just be silver
Human hemoglobin is composed of iron which acts as an oxygen carrier. This is why when blood is oxidized it turns red (rust).
Silver is also a metal which easily oxidizes. I believe it could be used in replacement of iron in the blood. Because it has a smaller ionic charge than iron you would need more. Depending on the quantity their blood could actually look shiny silver when unoxidized and tarnish when oxidized.
Would be interesting having people hunt them for their blood.
Nickel could also be used:
Just as iron and silver it can be oxidized. When unoxidized it appears shiny kind of like silver when oxidized it just dulls slightly.