Bianchi is the plural of whites. "People of light complexion" is a good guess, but not an educated one. A religious cult called the "Bianchi" because they dressed in white robes did develop in Italy about the time the family was named, but it seems these people did not take Bianchi as their surname because it had already been taken by members of a political group: When the Ghibelline, who supported the Holy Roman Empire (with its German Emperor), were banished from Florence in the 14th century, divisiveness continued. The Guelphs, victorious against the Ghibelline in their struggle for local autonomy, split into the Bianchi and the Neri (the whites and the blacks). The Neri then succeeded in banishing the Bianchi (including the writer Dante Aligieri).
I have read that my own Bianchi ancestors had previously been known, in 3rd century Rome, by the name of Plantus.