本为戏曲用语，现喻指不顾及人情关系公正办事to act justly with no regard to personal feelings; to play the role of a just character in an opera

• 荆州地区审计局为企业挡驾唱黑脸。

  The audit bureau of the Jingzhou region acts justly and impartially for the sake of fair business.

• 从严治军有时会得罪人，但如果需要，我来唱黑脸。

  Keeping strict order in the military can sometimes mean stepping on people’s toes, but if necessary I can be an impartial judge.