Try setting the meshes to 2-sided? The blotches mainly appear a bit below each end portion of the planks, so it could be an amount of light is not getting absorbed / bounced just inside the end portion, resulting in the blotches. I also notice the lighter the plank is in the picture, the lighter the blotch…and easier to see the completely black spot such as the 4th plank where there’s a big black spot right-of-center.