Since you are writing about a book, use the same terminology as the book uses, unless it is blatantly and patently offensive. If Steinbeck use "blacks", then use it, if he uses "people of color" (highly unlikely), then use that. The only terms I would avoid are "nigger" and the host of lesser known perjoratives.
With that in mind, the following are all acceptable: "black", "African-American", "Afro-American", "Negro".
One more thing: when you
quote directly from the text, quote it exactly. Mark Twain's "Nigger Jim" should never be replaced with "Afro-American Jim", much less the more formal "African-American James"
If your author says "blah blah blah nigger blah blah", then quote "blah blah blah nigger blah blah", don't quote it as "blah blah blah person of color blah blah."
Now, to throw one more wrench into the mix: the term "black" doesn't even necessarily mean a person of African descent. It would be very rare for a North American to use it in any other fashion, but Australians have used it for Aborigines, and many Brits used the term for dark skinned denizens of the Indian subcontinent up until the post-Colonial period.