You can set the sampling mode to NEAREST, BILINEAR and TRILINEAR via property for the Plane behavior. I just noticed you can't yet for Box/Cylinder (defaults to NEAREST atm) so it will be added in the next update.
If you want a really blurry look for some reason (N64 chique?) you can also try using FXAA.
So you are saying the fog color is grey and only appears on that model? Since boxes or any behavior meshes cannot be excluded from fog I am surprised by this outcome.
What's the light setup of the scene? Probably best to share the capx if you can so I can properly see for myself.
