On my old CRT I wouldn't have seen this, but now I'm on an mac with a better screen. It might be a calibration issue, but your darks (cave holes) (28,28,28) (26,20,34) (25,21,26) looks kinda of light, with the dark grey being lighter than the purples. I had to curve the image a bit to make things pop, but I have a preference for popping things.
Anyways, if you can't see the difference between 0,0,0 and 10,10,10 on your monitor you may have a calibration problem.
Edit: or is the dark grey just marking a parallax layer?