Couple questions. First, when we use a flashbang, how do we determine if someone is blinded by this? And secondly, if one of the heroes has a supersense activated are they more susceptible to be blinded?

When determining if someone gets blinded, the attack roll has to match/surpass the target's Fortitude (Pg. 72. Perks: Secondary Effects).

The supersense is susceptible only if that particular sense states so or the GM calls for it (pending on how the supersense works).