Who or what challenged negative ideas about african americans?

Well, there could be a lot of answers to that question.  Civil rights leaders like W.E.B. DuBois, Martin Luther King, Jr., and Malcolm X certainly battled against negative stereotypes regarding black Americans.

So did black athletes who showed themselves every bit the equal of whites, such as Jesse Owens and Jackie Robinson.

Whole groups of black Americans showed themselves to be capable and fit as citizens and heroes of the United States, such as the Tuskegee Airmen, the first black aviators in the US military, who fought for the nation in World War II.

Your question is a broad one that could have many possible angles for the answer.

The answer is protestant reformation.