The blue skin is caused by a condition called methemoglobinemia. Methemoglobin causes the red blood cells to carry iron in its ferric form rather than the ferrous form, which reduces the ability of the cells to carry oxygen to tissues. If the methemoglobin concentration is greater than 15%, a change in skin color and blood color are likely to be observed. In addition to the recessive trait carried by the Fugate family, other conditions may cause methemoglobinemia. The most common cause is G6PD deficiency.