There is not a lot of information about the Bianchi surname. Based on what I found below there are two possible origins. There are many branches of the name or family, with many Stemmari ( Crests ) recorded by Stemmario It.
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 thisi 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).
There is also a town in Calabria — Bianchi, so it is most likely that some people with this surname got there name from there.
It has been written that some Bianchi ancestors had previously been known, in 3rd century Rome, by the name of Plantus.