The answer is really quite complicated. There is less red light reflected from the shallow area between the surface and the vessel than blue light because blue light is scattered more. The surrounding tissue reflects back more red.
This does not make the vessel blue, however. It only makes it look blue. The authors had to use Land's retinex model of color vision to show that the contrast between the two areas of the skin are interpreted by the brain in such a way as to make it look blue.
This is only my interpretation of a pretty complicated paper. Now I have to figure out how to explain it to my brother :)
Now if this really a retinex effect you should be able to do a pretty easy experiment. If you eliminate the surrounding color the blue effect should go away. I just took a piece of white paper and punched a round hole in it with a three-hole-punch. When placed over a vein, sure enough I don't see blue. Try it yourself.
Mark Lund