What you're seeing is AA(Anti-aliasing)
AA is quite a nice thing, but doesn't work with voxels in a way that is beneficial. Just find your voxel models texture in unity. Select it and look at the inspector. You should see texture options. Basically just set the filter mode to point and you'll be done. I also recommend you use truecolour format.
Do this for all imported voxel textures.
Here is a picture of how I set my textures up for voxels.