Yep, Squiggly_P applied it nicely and wisely. I'd suggest to drop the "blue bar" before the dithered shadow, though, and instead use that blue to "underline" walls, as if they had a special footstone that reads as "look, this is the bottom".
I initially planned to suggest something like Konrad's edit, but i wasn't convincing... at least, it doesn't work as well as it does in Zelda perspective and "black-because-you-can't-see-what's-here" works better.
Make sure you're consistent, though: your treasure chest currently use a different perspective and that might break the illusion.