The labís explanation for the black spec is ridiculous.

Hereís why. Consider a tiny point A of the subject. Light reflects in infinitely many directions from A to EVERY point of the front surface of the lens. Light rays from each of these points are refracted by the lens assembly and directed to the single point B on the film that corresponds to A.

A spot on the lens will block only a tiny percentage of all the rays that form the image of A at B. The loss of a few rays wonít make any meaningful difference in the image on film.