Blood found in humans is red. Human blood is made of red blood cells containing hemoglobin. Hemoglobin is a protein containing iron and is red in color. Hemoglobin aids in the transport of oxygen.
When human blood is oxygenated, it is bright red. Blood appears dark red when the hemoglobin is de-oxygenated.
Veins appear blue under the skin because of the way light is absorbed and reflected back through the skin. The thinness of the walls in veins makes a difference in the way the light is absorbed. Arteries are made up of thick walls of muscular tissue, which also make the light reflect differently.
Spiders, molluscs and a number of anthropods have blue blood.