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. hope this is correct ^^

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