"Chiaroscuro" is an art term describing a high contrast between shade and lightness in a painting or drawing. Chiaroscuro, the rat, was named this by his parents because they liked to have fun and be a little joking in naming their babies. "Chiaroscuro" is a funny name for the rats because the rats of The Tale of Desperaux all hate light and spend their lives in the shadows-- to even name a baby rat something to do with light is laughable. But when Chiaroscuro grows up, he becomes obsessed with light. Though he spends much of his life in the dark shadows of the dungeon, he seeks out light at every opportunity. In this way, he comes to embody his name in his manners.

The nickname "Roscuro" is simply a shortened form of his full name. The nickname does place more emphasis on the Italian root -oscuro, meaning shadow, which might have been deliberate on the author's part. For Roscuro to love light so much but be forced to spend his time in the shadows, it is similarly fitting that his name emphasize the weight shadow has on his life.