I'm looking for a sort of "shadow" block that dims/tints light that passes through it but has little or no visible texture of its own and the player can move through like air, something that can be used to create a good "spooky dark tunnel" effect without making everything darker everywhere

I don't know how possible this would actually be, but there are already things that block liquids but not players or light, players but not liquids or light, and players and liquids but not light.

Heck, I think some liquids even block light but not players, but then you have to deal with the whole hassle of keeping the stuff contained and not spilling all over the place or draining into the background (which can be done, but requires you to have a liquid that blocks light without obviously looking like a mass of fluid, which I do not know where to find)