It’s a neat trick, creating something both slavishly retro and distinctly modern. Dan Auerbach manages just that with his blues-based rock duo, the Black Keys. While he stays true to the essence of the music, he’s not hesitant to scoff at tradition. In the producer’s chair for the third album from Cadillac Sky, Auerbach takes the same attitude. So does the band. Led by Fort Worth native Bryan Simpson, Cadillac Sky once looked, from a distance, like just another talented young bluegrass band. But something happened on the way to the hoedown. Letters in the Deep (Dualtone) sports banjos, mandolins, and fiddles, but it ain’t bluegrass—not by a long shot. Infused with impassioned singing and rock and roll vigor, the album isn’t about picking. It’s about songs, some of which—“Hypocrite,” “Hangman,” “Ballad of Restored Confidence”—explore dark passages, showing real grit and staying power. The stringed instruments, recorded mostly live in Auerbach’s analog studio, take on a strange, otherworldly feel, and when the moment calls for it, the band isn’t afraid to get a bit riled up.