They had hereditary methemoglobinemia caused by an absence of the enzyme diaphorase from their red blood cells.

Methemoglobinemia is a rare hereditary blood disorder that results from excess levels of methemoglobin in the blood. Methemoglobin which is blue is a non-functional form of the red haemoglobin that carries oxygen. It is the colour of oxygen-depleted blood seen in the blue veins just below the skin.

In normal people haemoglobin is converted to methemoglobin at a very slow rate. If this conversion continued, all the body's haemoglobin would eventually be rendered useless. Normally diaphorase converts methemoglobin back to haemoglobin.

This condition is inherited as a simple recessive trait. In other words, to get the disorder, a person would have to inherit two genes for it, one from each parent. Somebody with only one gene would not have the condition but could pass the gene to a child.

The antidote for this condition is to give the body an alternative method of converting methemoglobin back to normal. Activating it requires adding to the blood a substance that acts as an "electron donor." Many substances do this, but methylene was used to treat the families in Kentucky.

See: http://www.rootsweb.com/~kyperry3/Blue_Fugates_Troublesome_Creek.html