"list of persons who have incurred suspicion," 1610s, from black + list. Specifically of employers' list of workers considered troublesome (usually for union activity) is from 1888. As a verb, from 1718.
Concerted action by employers to deny employment to someone suspected of unacceptable opinions or behavior. For example, individual workers suspected of favoring labor unions have often been blacklisted by all the employers in a region.
Note: During the McCarthy era (see Joseph P. McCarthy) in the 1950s, the careers of many public figures suspected of communist activities were ruined by blacklisting.