What causes the blue pigment of indigo to appear in blue diaper syndrome?

In blue diaper syndrome, the blue pigment of indigo appears due to the presence of an excess of tryptophan and its metabolites, particularly indole. When indole is metabolized in the gut by bacteria, it can be converted into indigo, which is then absorbed into the bloodstream. This indigo is excreted in urine, giving it a characteristic blue color, and can also stain diapers blue. The condition is often associated with a defect in the transport of certain amino acids, leading to elevated levels of tryptophan.