It averts the eyes. If you stand in the mirror naked then again with a tight fitting black dress you will notice how you seem thinner. Everything that is black or in dark color has a slimming effect. It's kind of like stripes. If you wear stripes that go side to side you appear wider where as stripes going up and down cause you to seem thinner. They make your body seem longer. It's a simple trick of the eyes.
White reflects all of the light that hits it so every imperfection is highlighted. Black absorbs all light so it has a masking effect and it is harder for the eye to distinguish the different curves and lines of the body creating a stream lined look.
I do believe that wearing black will help you to slim your body further as it absorbs the heat and retain the heat that helps you to burn out fat. It is similar to the principle of those who go for sauna.
Form fitting black hides the shadows, dimples, lumps, and bumps while light colors -OR- loose BLACK clothing that lays against bumps just defeats the purpose. secondly, Dark colors (like rich dark chocolate) relaxes them awwwwaayyyyy.....
I must say that in my eyes black makes you look slimmer because it hides the visible shape of your body and make u beautiful in others eyes and in comparison of white its because white reflect all of your body parts in lights