The generation of iris color is quite complex and the exact color of the eyes depends upon many different factors, not just melanin. In fact, blue coloration does not depend on a specific chemical, but on the structure of the eye itself.
This very nice review (free) summarizes our current understanding:
Genetics of human iris colour and patterns - Sturm and Larsson, 2009
From the Introduction (bold is mine)
In the brown iris there is an abundance of melanocytes and melanin in the anterior border layer and stroma whereas in the blue iris these layers contain very little melanin. As light traverses these relatively melanin-free layers, collagen fibrils of the iris scatter the short blue wavelengths to the surface, thus a blue iris is a consequence of structure not of major differences in chemical composition. Different shades of blue, and in irises with a limited amount of melanin, different shades of grey, green and hazel, are determined
by the thickness and density of the iris itself and the extent of accumulation of white collagen fibres, as well as patches of tissue loss in the anterior border layer and stroma.
However, a careful examination of people’s irises makes it clear that there are characteristics other than eye colour that present in the human iris. The iris has been analysed to show that it can display a degree of complexity encompassing over 240 degrees of freedom (Daugman, 2003).