I really like cave tilesets. Partially because there are so many different ways you can interpret them in. But on top of that, most of these ways haven't been used yet. Yours right here looks really sweet. I like the shading on the center part and how they interact with the corner colours.
I've decided to change up the level's artstyle entirely. NPCs, BGOs, background will all have their own shading style. While the blocks have black outlines, background objects will have a white one.