A (pejorative) term for a company that exploits employees (e.g. forces unpaid overtime, engages in illegal labor practices); essentially the same as ブラック企業.
彼は入社してすぐに社風が合わず、ここはブラック会社だと上司に相談した。
He didn't fit the company culture soon after joining and told his boss that this is a black company.
求人の条件が良く見えても、口コミでブラック会社かどうか確認した方が安全だ。
Even if a job listing looks good, it's safer to check reviews to see if it's a black company.
労基署（労働基準監督署）に相談して、ブラック会社の実態を調べてもらった方がいい場合もある。
In some cases it's better to consult the Labor Standards Inspection Office to investigate the reality of a black company.