I've never expected the game will be the 'darkest Zelda ever', whatever that means. Myself included, I think most people feel like just from the first teaser alone, the game is already darker than Botw was.
Botw had it's focus on nature, and despite the calamity at the castle, not much really felt foreboding. Even the blights didn't feel much like evil entities, despite their looks. Rather, they were more like forces of nature. For this game, I think they're going to put more emphasis on dark and light themes in equal parts.
It seems like the overarching theme is similar to what they did with the Ancient Cistern, which is based on the story of 'The Spider's Thread'. The layers of the world, sky all the way to the underground. The sky seems divine, bathed in golden light, has prominent lotus symbolism and Buddist architecture. And I suspect the underground will reflect hell, as it did in Skyward Sword's Cistern.