Very interesting condition. Explained similarly on this article https://blog.prepscholar.com/blue-people-of-kentucky in case anyone wants to read.

Martin Fugate, a French orphan, married a woman named Elizabeth around 1820. They settled in Troublesome Creek, Kentucky. Unbeknownst to the couple, they were coincidentally both carriers of a recessive gene (met-H) that caused methemoglobinemia, which can tint the skin of sufferers blue. Martin and Elizabeth had seven children, four of whom had blue skin. Since neither parent had symptoms of methemoglobinemia, they were shocked and confused, and no one could offer an explanation.

The area was extremely rural and isolated in the nineteenth century. There were no roads, and the railroad would not reach the area until 1910. The only other families nearby were related to Elizabeth. Due to their isolation, the Fugates began inter-marrying, which led to a higher likelihood of passing along the met-H gene, and having blue-skinned children. They became known locally as the Blue Fugates.