Veins appear blue because light, penetrating the skin, is absorbed and reflected back to the eye.
In an experiment, glass tubes were filled with blood and immersed in milk, milk having a similar ratio of fat, proteins, and water in emulsion as skin. At a certain depth, the tubes appeared blue. Almost all samples were dark red. Those that weren't were usually because people were on enriched oxygen systems. At the same time, respiratory therapists drew blood from an artery, not a vein.