I see this sentence using tenebrous in a simple simile. "Cherry blossoms...like tenebrous snow." Like dark gloomy snow in the glow of lamplights. The character is trudging through cherry blossoms that feel like the burden of "dark" snow. To me, it's not a contrast at all. The writing sets a mood for darkness, gloom, and death; all of which represent the main character of Hard Rain, John Rain.

Now, I haven't read this book yet, so I don't know what context this sentence is used in, but I did read the previous book Rainfall which is how I know about the character.