The masculine singular 'bianco' and the feminine singular 'bianca' are spelled with just a C. However, if you spelled the plurals as 'bianci' and 'biance' the "k" sound of the C would change to a "ch" sound (because in Italian, 'ci' or 'ce' is pronounced with a "ch" sound: "bee-an-chee" and "bee-an-chay"). Therefore to maintain the "k" sound you add an H to make 'bianchi' ("bee-an-kee") and 'bianche' ("bee-an-kay"). :)
It's in order to preserve the /k/ sound. > "bianco" and "bianca" have a /k/ for the "c" (because "o" and "a" are back vowels) > The added "h" in "bianchi" and "bianche" is a buffer so that "c" will keep the /k/ sound, since "c" followed directly by the front vowels "i" and "e" would lead to a /tʃ/ sound (palatalization). * That way, all 4 forms maintain a more similar pronunciation (just the crucial final vowel varies, for agreement).
I may be wrong, but here it is: shouldn't the word camicie be spelled camice? There are two logical options here: 1) We insist on keeping the penultimate i in camicia, spell the plural camicie, and pronounce it ca-mi-ci-e 2) Keep the current pronuciation ca-mi-ce, and spell it the same: camice.