It’s not just horseshoe crabs – other crabs and lobsters have blue blood, as well as most molluscs.
Our blood is red because it contains haemoglobin. This is a red molecule because it contains iron atoms that have a red colour when they bind to oxygen.
Crustaceans don’t use haemoglobin, but have a different pigment called haemocyanin in their blood. This uses copper atoms instead of iron to absorb oxygen and copper has a blue-green colour when it oxidises.