We might mention "Patches" (not the Clarence Carter song, the Dickey Lee song), "Ode to Billie Joe" by Bobbie Gentry or "Sukiyaki" (Ue o Muite Arukou) by Kyu Sakamoto which were dark in the 1960's but by-and-large I agree that modern songs are darker on average.
One of my least favorite songs of all time because the protagonist is a credulous poltroon. But a year later, an antidote to that song came out featuring a protagonist who was courageous, focused and motivated.
Midnight Mary--Jerry Cole (1963)