A little off topic but this is another cool thing to do related to what Ian C wrote above.
A lens can 'see through' any object placed right in front of it as long as the aperture is wider than the object. Put a pencil right in front of the lens. You can 'see through' the pencil until you stop down to the point that the physical aperture size equals the width of the pencil.
With respect to the original post, I'm not sure I follow. Is the defect on the negative the size of a pin hole or smaller? Or does it look like a worm or a fiber?