Why the color motion picture film, or a color photographic slide, appears as black? The film appears black but when the light from the projector lamp hit it then you can see all the colors.
Because the projector's light is so bright, to give a maximum brightness appropriate to rendering bright objects, like reflections of the sun, or the sun itself, in the film. So the film has to absorb most of the light from the projector, making it very very dark.
The brighter the light the greater the range of the film brightness, but the darker the film has to be to give normal brightness.