Why does sunlight cause plastics to become a faded blue color?
I don't think its actually blue, I think the blue is a result of Rayleigh scattering.

Over time the dyes are destroyed by light removing any color. simultaneously the plastic is degraded and becomes rough on the surface and acquires an inhomogeneous refractive index within the structure due to the destruction of bonds and oxidation and the end of chains. These two structural changes cause scattering, and since blue is scattered more than the other colors due to the relation of scattering with wavelength to the fourth power, the material appears blue when looking at the reflected light.  If you look at transmitted light going through the plastic it should appear yellow.