Bruce Hornsby will do a lot of things—join the Grateful Dead , write a hit with Don Henley, celebrity-coach the UVA basketball team—but he won’t do blow. In “Silhouette Shadows," a nearly six-minute song from his lively new album, Indigo Park , the 71-year-old songwriter enters a rare kind of flow state with an intricately composed melody that somehow feels improvised and lyrics that sound stream-of-conscious but build to something quietly profound. Weaving between his piano lines, he recalls watching Nixon resign through somebody’s window (“Good thing I’m tall”), feeling cognitive dissonance after his fellow classmates celebrated when JFK’s assassination resulted in a day off from school, and taking a tell-tale journey to meet a “big-time producer” and declining an offer of cocaine: “Instant ostracism," he sings sadly. The anecdote comes off less like an afterschool special and more like a reflection on the strange in-between zones where Hornsby has always thrived.…