Obama is a real person whose racial identity is known and tied to history, Odysseus is part of an oral tradition that changed many times before it got to be written down (here's a really interesting review of the various English translations from the Greek). Nolan has taken a universal story - Hero sets out on a quest, faces challenges, has a helper, is changed in some way, returns home - and used it to explore things that interest him: loneliness, what happens to morality when civilisation breaks down. Blind-casting is a bit of a red herring given it's a story, but anyway, the ancient Mediterranean was a melting pot of diversity and inter-relationships.