Black males, overall. Abigail K. Wozniak has a new NBER paper on this topic:

Nearly half of U.S. employers test job applicants and workers for drugs. A common assumption is that the rise of drug testing must have had negative consequences for black employment. However, the rise of employer drug testing may have benefited African-Americans by enabling non-using blacks to prove their status to employers. I use variation in the timing and nature of drug testing regulation to identify the impacts of testing on black hiring. Black employment in the testing sector is suppressed in the absence of testing, a finding which is consistent with ex ante discrimination on the basis of drug use perceptions. Adoption of pro-testing legislation increases black employment in the testing sector by 7-30% and relative wages by 1.4-13.0%, with the largest shifts among low skilled black men. Results further suggest that employers substitute white women for blacks in the absence of testing.