Our entire system of perception is based on contrast. The blacks are the foundation of an image in that everything else plays against them. Red looks redder when there are deep blacks in frame, because our eyes have a reference for not-red. Make sense?
Images with weak blacks tend to look more washed out. Some cinematographers like and use this effect. For example, take Harris Savides, who has been known to use low-contrast film stocks which he then underexposes, sometimes also using older lenses with more veiling glare. Depending on the colours in the respective scene, the blacks in these films (e.g. Birth and American Gangster) tended to take on a lighter, dark greyish or brownish tone, which I would say is more naturalistic than the solid blacks that technical perfection demands. In life, it's quite rare to see a pure black.
Obviously, the nature of the film suggests the cinematographer's approach to the blacks, as with every other aspect of the images, and people have different ideas as to what constitutes naturalism even if that's the approach the film seems to want. And there's no right answer. Whatever you think looks better, is better, as long as it's right for the film.