The genetic mutation can occur when a person has a deficiency in the enzyme called cytochrome-b5 methemoglobin reductase, which is responsible for recessive congenital methemoglobinemia. They also lacked the enzyme diaphorase in their red blood cells, normally diaphorase converts methemoglobin back to hemoglobin.
Methemoglobinemia is a blood disorder in which an large amount of methemoglobin (a form of hemoglobin) is produced
Methemoglobin can develop blue skin without any negative symptoms.
Meet the Fulgates, a family of people with blue skin that lived in Kentucky for many generations. Most of the family members never suffered any health effects and lived into their 80s and 90s. It was discovered that there was inbreeding within the family that contributed to the high number of blue offspring.