[Michlib-l] book character

Does anyone know what book this might be? I had a patron tell me he's been looking for this book for years, but doesn't know the title, the author or even if it's fiction or non-fiction. It takes place in Vietnam and one of the people is a Staff Sgt. nick-named "Sgt. Blue" because his skin was such a dark black that it had a bluish hue. I've searched and can't figure this one out. Anyone have a clue to what this book is? Thanks--Glen