This is off the top of my head. I'm not sure it is right but it seems reasonable to me.

Blue color is caused by colorless crystals of quanine in the cells named iridophores or quanophores. The crystals refract the blue portion of the light that falls on the skin. Melanin absorbs light. If the melanin is reduced or absent, then more of the refracted blue light can be seen, giving a bluish to purplish effect.

Hope this helps.

Paul Hollander

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