Ask yourself: What do the words “Black Power” signify to you? That’s the question several of the Mangrove Nine — nine Black activists arrested when a public demonstration against London police harassment on Aug. 9, 1970 devolved into an incendiary example of the very thing they were protesting — put to each and every one of the potential jurors in what would prove to be a landmark civil rights trial. It was a savvy strategy, focusing participants’ attention on what the accused felt their defense was really about: racial justice. Steve McQueen doesn’t overtly repeat the group’s jury-screening query in “Mangrove,” the powerhouse courtroom drama that kicks off his upcoming “ Small Axe ” anthology series for Amazon : five standalone films designed to explore and elevate dimensions of Black life in Britain — from music and food to family and romance — set between 1968 and the mid-1980s. And yet, taken in toto, the