Skybox prop and displacement lighting

I'm using rock props and details on displacements in the skybox in several maps, and I've always had odd lightning with them - rocks are lit by ambient lighting from beneath very strongly, and displacements are nearly pitch black when facing away from the sun cast direction (I have tree materials on displacements).
I've tried nodrawing the bottom face of the skybox to no avail, as well as putting a blocklight brush under the map in the skybox, yet prop lightning doesn't seem to change at all. I can't think of any way that I might fix the displacement lighting either, short of using ghost omnilights/spotlights - is there any better way?

Edit:
I've fixed the displacement lighting by turning them into propper models and setting 'ignore surface normals' - ambient lighting on the rocks is an issue still, though

Pocket said:

It would help if you posted some pictures of what you mean. Lighting can go haywire if your map is leaking, and the business with the props might be a symptom of not using -staticproplighting, but without pictures I can only take a stab in the dark.

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Apologies, I completely forgot about screenshots. I always compile with -staticproppolys/etc and there isn't a leak, the issue is just that I've got strong ambient lighting on some props. I've partially mitigated the issue with displacements underneath them but it hasn't solved it completely. The first image shows a prop with the blue tint from underneath, while the second shows what one of my rocks without the issue looks like.
I can upload more images if these aren't quite clear enough.

Uncuepa said:

the blue lighting from underneath is from the skybox, you have that large gap where your map is, and so the skybox is filling that space with ambient light - if you want to avoid that blue tint, you have to block the light from underneath. I'd suggest a nodraw or toolsblack texture layer

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That doesn't seem to change anything unfortunately - even with the brushes right up under the props, there's no change in the lighting