Unions and Discriminatory Practices
African-Americans have the highest rates of unionization. Part of the reason is because this demographic group is highly-represented in the public sector, the industry with the highest rate of unionization.
Another plausible reason is that unions help fight discriminatory practices by employers. Evidence from New Zealand provides some evidence for this. Here is the link to the paper. Here is the abstract:
This study examines the effects of unions on employer compliance with antidiscrimination legislation in New Zealand, using a sample of 227 employers. The results do indicate that unions do reduce discriminatory practices. More specifically, higher levels of unionization do increase the level of employer compliance. However, other union characteristics, such as union size and strike propensity, appeared to have no influence on employer practice.