While Dustfinger is described as having "dark skin" in the book, his ethnicity is never explicitly stated. It's more important that he's a character who is "inked" out of a book and struggles to adapt to the real world.
It's worth noting that Paul Bettany is of English descent, and the film's casting of him as Dustfinger has been criticized for perpetuating stereotypical portrayals of people of color.