An octopus’ blood is blue due to the protein called, haemocyanin, which carries oxygen around its body, contains copper rather than iron like we have in our own haemoglobin. The copper-based protein is more efficient at carrying oxygen molecules in cold and low-oxygen conditions (i.e. ocean life).
[https://www.nhm.ac.uk/discover/octopuses-keep-surprising-us-here-are-eight-examples-how.html](https://www.nhm.ac.uk/discover/octopuses-keep-surprising-us-here-are-eight-examples-how.html)
If the blood (called haemolymph in invertebrates) becomes deoxygenated – when the animal dies, for example – it loses its blue colour and turns clear instead.