Jazz thrives on humility, which may explain why so few people seem to care about it nowadays, but director Alan Hicks sure as hell does. Hicks’ documentary on his former mentor, nonagenarian jazz trumpet hero Clark Terry, is a reverie of lucid gentleness for its own sake. Such states of grace can exist in music, as Terry’s generous career attests, and also in teacher-pupil friendships, like the one we see here between Terry and his recent protege Justin Kauflin, a sweetly striving pianist in his early 20s who happens to have been blind since sixth grade. As jazz-coach movies go, Keep On Keepin’ On is pretty much the anti-Whiplash: candied with affection, decidedly downtempo, and tender to a fault. Expository due diligence calls for an introductory roundelay of jazz eminences assuring us how great and important Terry is, but that’s self-evident from archive clips of him blowing brightly over the course of 70-plus years. It’s easy to hear why his lifetime-achievement Grammy came with an ovation of gratitude for having “the happiest sound in jazz.” The film follows Kauflin’s struggle to come of age, musically, while Terry’s health deteriorates. We’re made rather anxiously aware of the master’s mortality — and by extension the closure of a cultural era — but also deeply reassured by the enduring wisdom that he imparts, and the joy.