If, as many on the Left seem to think, it's okay to tar today's conservatives with the sins of conservatives in previous generations, why does the same not apply to Unions? Consider the motives behind the Davis-Bacon act, from a review of David Bernstein's Only One Place of Redress:
Depression era legislation, though officially colorblind, was often highly discriminatory. A case in point was the Davis-Act requiring construction firms with federal contracts to pay "prevailing wages." As defined by the Department of Labor, the prevailing wage usually equaled the union wage thus freezing low-skilled black workers out of many projects. As Bernstein points out, "contractors had every incentive to hire unionized workers for skilled positions. Union members were generally the best-trained workers, and they could be hired quickly and efficiently through union hiring halls." Many backers of Davis-Bacon did not hide their racist goals. The testimony at the hearings for the bill by William Green, the president of the American Federation of Labor, was a clear example. Green praised the proposed law because it would make it more difficult for contractors to "demoralize" wage rates through use of low-wage "[c]olored labor"
In general, in American history, American Unions have not covered themselves in honor on racial matters.