Firstly, to keep things simple, I will assume a straight male sexual masochist who enjoys being made to suffer, in some way, by a woman that he is sexually attracted to. This is perhaps the most common scenario that happens with regard to sexual masochism. Of course women can be sexually masochistic, as can gay people, both gay men and gay women. But I will use the example of a sadomasochistic straight man here in this article, so that I can say "he" and "she", without endlessly repeating the permutations of who does what to who and who is attracted to who.
This situation, where a man wants to be made to suffer by an attractive woman, can be described in terms of distraction. The man wants the woman, but he also wants to be severely distracted by something that the woman does, so that he can enjoy the excitement of ignoring whatever it is that she is doing to him.
So, how does this relate to shiny black clothes?
When a man looks at a woman dressed in shiny black clothes, he sees the shape of the attractive parts of her body, but he sees them covered in a surface with particular optical qualities.
The most important optical qualities are:
Matt reflection can be defined as reflection where the perceived color and brightness is not a function of viewing angle. Conversely, gloss reflection is where the perceive color and brightness is a function of viewing angle more than anything else.
To arrive at a theory of distraction, we need only assume that the sexual instincts surrounding the appreciation of the female form are contingent only on the observation of the woman's three dimensional shape via the perception of matt reflection.
So what's happening, when a man sees an attractive woman wearing shiny black clothes, is that perception of the reflections due to the glossy component of the material's reflective quality is a distraction from the sexual stimulus, which is entirely contained within the perception of the matt component of the material's reflective quality.
This theory of shiny clothes fetishism does make one simple prediction: that if the matt component of reflection is reduced too much, i.e. if the material is too black, or the lighting conditions are too dark to see any matt-reflected light, then the sexual effect will disappear, or at least be much reduced.