Blue

You may know the blood of octopus is blue in colour.

Scientifically, octopus blood contains hemocyanin, a copper-rich protein, for oxygen transport. Although less efficient than the iron-rich hemoglobin of vertebrates in terrestrial environment, hemocyanin oxygen transportation is more efficient than hemoglobin in cold conditions with low oxygen pressure.

Maybe that's the reason why in his classic novel The Man With Blue Blood (藍血人), Ni Kuang (倪匡) mentioned that these beings - although look exactly like human - come from Titan (the largest moon of Saturn).

Of course, blue blood has an entirely different meaning in modern English.

But, no wonder many who claim themselves blue blood are invertebrates.

PS. Unlike hemoglobin, hemocyanin dissolves in plasma and puts less workload to the heart (because the blood is less viscid). We need our hemoglobin confines within red cells because the molecule would bind to nitric oxide (chemically very similar to oxygen) - which is crucial for blood vessel dilatation.