Why have black people in the US traditionally voted for Democratic candidates?
In the last eighty years or so and outside of the deep South, the democratic party has had a better civil rights record. This has not always been the case. Following the Civil War, newly enfranchised African Americans voted overwhelmingly Republican, the party of Lincoln who freed them from slavery. This disappeared at the end of Reconstruction (occupation of the former Confederacy by Federal troops) as these states passed various laws which disenfranchised the former slaves. By the time these discriminatory laws were repealed, the Republican Party had shifted in the political spectrum to conservatism; not in the political interests of the majority of African American voters. In recent years, many affluent African Americans have shifted allegiance to the Republicans, as it is in their economic self interest to do so.
