The blue coloration of the iris is due to what?

The blue coloration of the iris is due to low concentrations of melanin in the stroma of the iris and because fibers on the fronter layer of the iris scatter and absorb some of the longer wavelengths of light.

Explanation:

Iris is thhe colored part of the eye and it is made up of two layers: the "back layer" called the pigment epithelium has brown pigment in it while the "front layer" or stroma contain overlapping fibers that might or might not have pigments in it (e. g. brown eyes have pigment melanin even in the front layer of the iris).

Eye color inheritance is a polygenic trait, meaning that it is controlled by the interactions of few genes.