Hi MAXAX!
I can't remember exactly,but i think there are two reasons:
1. The refraction of this material lets the spectrum's blue light go through.
2. Black light is able to lift atoms of this polymer on a higher level. This atoms will fall back at once and emitt energy, causing this effect. But i don't know the formula of this polymer. I think the idea of Ibanez, to use this polymer as a pickgard is very cool.

Wolfgang