Sorry. Yes I mean the dark outer edges. In all of my tests I have not been able to get rid of this.
This material has 2mm attenuation, 128 gray attenuation color and 128 gray scattering color. The box is just a newly generated box, the other geometry was re-smoothed using 'auto-smooth'
This effect remains even when the corners have been chamfered, it seems like the problem is consistent with sharp corners in general