Okay, now here's something fascinating.
Apparently there was a whole... well, clan of people living in Kentucky who had a rare hereditary blood condition that turned their blood very dark brown and their skin blue, much the same way our skin is blue when the veins are near the surface. This is a very recessive trait and the four families who were affected by it only got the disorder because their ancestor, Martin Fugate, was a carrier and he happened to marry a woman who was also a carrier, which was a massive improbability but happened anyway. Four of the Fugates' seven children were blue. Normally the gene pool would have reabsorbed the abnormality and they wouldn't have been blue, but it was a tight-knit community of only four families and they intermarried a bit, getting a reputation in the area as the "Blue Fugates."
Thanks to the wonders of modern travel, the Fugates aren't intermarrying with others who have the genetic abnormality at anywhere near the same rate, and the gene is in recession once again. Not that it matters, anyway, there's a pill that can fix it now. Can you imagine taking a pill every day for fear your skin would turn blue again? Plus the pill itself is also a blue dye, so it turns their urine blue.
This story is particularly interesting to me because my X-men stories feature a family with blue skin. I love when my fantastical ideas turn out to have scientific basis. It's a warm fuzzy feeling, much like how I imagine the fundamentalist archaeologists feel when they find evidence of the Great Flood. Except their fantastical ideas came from somewhere other than my overactive imagination...