What does the word Roscuro mean?

There is no Word "Roscuro' in French or any other language. Differently from 'Despereaux', which is French for 'frustration' or 'despair', 'Roscuro' is an adaptation the movie script writers made up, and the reason is this:

In the book, the character's name is Chiaroscuro, blend of 2 Words, 'chiaro', which means in Italian 'light' or 'clear' (from there we get the Spanish 'claro'), and 'oscuro', which means 'dark' or 'black' in all forms. No doubt this describes Roscuro's character, since he lived in a dark world, but wished for the light above. It also aptly describes his performance as a basically 'light' or good person, but now sadly fallen onto a dark path, until Despereaux helps him return.

Now from 'chiaroscuro', the writers lopped off the first 4 letters (chia), which leaves us with 'roscuro'. Hence, this was the form adopted for Despereaux's rat friend in the movie.