His grandmother would have been "too dark to pass." She had actually blushed. As though she’d discovered something shameful about him. (2.12.292)
When Milkman meets Susan Byrd and Grace Long, we have the first discussion of "passing," calling to attention the fact that, to escape the injustice, violence, and intolerance rampant in America, people would deny their heritage to live a more free life. Here we see the connection between skin color and shame in America.