That's because the external fuselage is not textured in the VC/Cabin. Your view is not supposed to move too far outwards, and certainly not outside of the VC/Cabin, so that you can see the exterior of the 757 (which is physically inmpossible in real life, unless the aircraft has serious damage), so when you move too far towards the outside, or even outside the VC/Cabin, you see a black wall (the external fuselage with no texture).