How does Kay Boyle use imagery in Black Boy?

Boyle uses intense imagery to refer to the character's blackness. The boy was "as thin as a shadow but darker" and his face was "black as a bat's wing, nodding and nodding like a dark heavy flower." Another black boy who pushes the carts has a face "dripping down like tar in the sun." All of these descriptions emphasize the extreme darkness of the skin color of these boys who work on the boardwalk.