Blue Skin

The Fugates family in Kentucky acquired a rare blood disorder called Methemoglobinemia that caused some family members to have blue skin tones. As detailed in the 1980s, the combination of a very rare and recessive genetic trait carried within a thinly populated area.

Paul Karason of Oregon used a home remedy that resulted in argyria, or silver poisoning, a permanent disorder that left him with very blue skin.

The Tuareg people in the Saharan deserts in North Africa wear clothing dyed in indigo that rubs off onto their skin, so much so that they are called the "the Blue People."

Krishna is a key figure in Hinduism and typically portrayed with blue skin in an allegorical context that evades me.