Thanks, I'll look into this soon

I was able to reproduce the issue. To fix the normals in the same version, disabling the "Compute surface normals" flag (interestingly) results in proper normals. You can do this in Segmentations module, by clicking Update on the Closed surface representation.

I'm trying to fix it so that the normals are fine with the flag turned on, as it's supposed to be.

Thanks. For now I'll stick to performing the smoothing with a very small factor, since it looks better and doesn't modify the model (too much?).

It turned out that the surface was inside out because the volume you attached is in a left-handed coordinate system. So if you just painted a sphere, and showed surface with no smoothing, it was also black. The solution was to detect this case, and if normals is computed by the marching cubes step (which happens when moothing is set to 0), then the normals are flipped as a post-processing step.

Fixed in https://github.com/Slicer/Slicer/commit/47d1643030d8c6e7cdb2953a8ad5acfee1836803

Thanks Csaba!