MINNEAPOLIS — Relief, even if fleeting and momentary, is a feeling that Black Americans have rarely known in America: From slavery to Jim Crow segregation to enduring punishments for living while Black, a breath of fresh air untainted by oppression has long been hard to come by. Nonetheless, the conviction of ex-cop Derek Chauvin for murdering George Floyd nearly a year ago allowed many across this city and the nation to exhale pent-up anxiety — and to inhale a sense of hope. But what might they feel hope for? The fate of Chauvin — found guilty of murder and manslaughter for holding a knee to Floyd’s neck, choking off his breathing until he went limp last May — showed Black Americans and their compatriots once again that the legal system is capable of valuing Black lives. Or at least it can hold one white police officer in Minnesota accountable for what many declared an unambiguous act of murder months ago. “This may be the beginning of the restoration of believing that a justice system can work,” said civil rights leader Martin Luther King III, echoing a sentiment that many expressed Tuesday. “But we have to constantly stay on the battlefield in a peaceful and nonviolent way and make demands,” he said.