As Maniac meets more people, he sees that not everyone is "color blind" the way he is. Why do you think Manic is "color blind"?
Part of the reason Maniac Magee might be "color blind" is that he has had such a difficult life. His parents died when he was very young, and he lived with his aunt and uncle, who hate each other. When Maniac finds Amanda Beale in Chapter 3, she is friendly to him when he asks her about the suitcase she is carrying. Maniac doesn't care that in the East End, all the kids are black. He thinks "He and Amanda and the suitcase were like a rock in a stream" (page 11). She provides friendship and stability for him in his unstable world.
Because he is running away from his old life, and he has never learned about racial prejudice, he doesn't understand the distinctions that other people make between black and white.
"Maniac loved the colors of the East End. The people colors. For the life of him, he couldn't figure out why these East Enders called themselves black. He kept looking and looking, and the colors he found were gingersnap and light fudge and dark fudge and acorn and butter rum and cinnamon and burnt orange. But never licorice, which, to him, was real black" (page 51).
The only thing Maniac cares about is that people accept him and are friendly to him, so he is essentially blind to the idea of a division between black and white.