Why the colour of blood of horseshoe crab turns blue in the contact with air?

The color of the fluid in the circulatory system of horseshoe crab is gray-white to pale yellow because the fluid carries little oxygen. The compound in the circulatory fluid of horse shoe crab that binds oxygen is comprised mainly of protein and copper, in contrast to iron protein complex in the hemoglobin in human red blood cells. When a horse shoe crab bleeds, the copper protein (hemocyanin) absorbs oxygen from the air, turning the circulatory fluid in the horse crab dark blue.