Why are arteries blue?
They are not.
Arteries and veins are neither red nor blue; they are a greyish-white, if one had to ascribe a colour to them.
Arteries transport blood that is bright red due to oxygenation, while veins transport blood that is dark red because of de-oxygenation.
Under the skin, because of the differential diffusion of red and blue wavelengths of light, arteries may appear purplish and veins bluish, but that is only an optical illusion