Why does Ned disparage black people?

Ned's partner, Pam, has children from a previous relationship with an African American man. As such, those children are mixed-race. They act as a constant reminder to Ned of his family's lowly status in life. In common with just about everyone at the trailer park, he still retains a sense of racial superiority, despite his extreme poverty and chronic drug addiction. Having biracial children doesn't make Pam herself any less immune to this attitude. She says that she'd rather stay in a motel than live in the "ghetto."
It's this widespread mentality among poor whites that allows Tobin to make so much money out of operating the trailer park. Conditions may be bad there—in fact, the trailer park's an environmental health hazard—but the residents would still much rather live there than in a black ghetto on the North Side. This means that Tobin is guaranteed a steady income stream from his white clientele, and he can hold the fear of ending up in the ghetto over the heads of anyone at the trailer park who might want to complain.