Rare Genetic Mutation
When families interbreed, it’s easy for their genes to mutate in these less-than-ideal situations. Their blue skin was most likely caused by rare genes that were combined with the interbreeding. Since they continued to interbreed, it only made sense for the gene to continue to mutate and get passed around. This gene can even be traced back to Fugate and Smith! As it turns out, it simply began that the two carried the gene and had children with it as well. Then, the interbreeding began. The son of Fugate and Smith, Zachariah, ended up marrying Smith’s sister (his aunt).