Why do veins look blue?
Well, even though most people presume that it appears blue (reference) is due to the blood being deoxygenated, it is actually not the truth because blood is never blue whether it is deoxygenated or not. In a paper by Dr Alwin Kienle titled Why do veins appear blue? A new look at an old question, the authors measured how much light of various wavelengths was reflected from both real blood vessels in skin and imitation vessels in a skin-like environment. The synthetic vessel (which was a capillary tube filled with blood and placed in a milky substance with optical properties similar to skin) allowed the authors to experiment with a variety of parameters (particularly vessel depth and diameter), and they were then able to validate their results by taking measurements on actual vessels in skin. They eventually came to the conclusion that the optical properties of skin and blood is what makes the veins appear blue. 

I quote from the paper as to the reason of blood appearing blue. 


  "Blue light does not penetrate as deeply into tissue as red light.
  Therefore, if the vessel is sufficiently deep, the reflectance in the
  blue will be affected to a lesser extent. Deoxygenated venous blood
  has a greater absorption coefficient than oxygenated arterial blood in
  the red spectral region, and this difference of two, rather small,
  values is amplified because of the long path length of red light in
  scattering tissue. As a result, veins are more likely to look blue
  than arteries at the same diameter and depth. Often arteries are not
  seen at all because they are generally smaller than veins and have
  thicker vessel walls. It has been shown that a small vessel will look
  red when close to the surface. However, if a superficial vessel is
  large it can still look bluish, particularly in the case of the vein.
  On the other hand, if the depth of a vessel is large, even red
  remitted light will not be influenced by the vessel, and it will not
  be seen."


So in short, the blue appearance could be attributed more to the location of the vein than just the deoxygenation.