Horseshoe crab blood is blue! After a storm, many crabs are left injured and bleeding on the beach. The blood contains copper which is what gives it that blue color.
Their blood is harvested by the pharmaceutical industry because it contains cells called amebocytes which are used to detect bacterial toxins. Testing manufactured products, such as vaccines and intravenous solution with an extract of horseshoe crab blood ensures that there are no contaminants.