My personal trainer Ruth, at Innovative Fitness asked me the other day if I would do a post on 'Why black is a slimming colour'. She said "Ever since we got our new uniforms which are these cute black zips, everyone has been asking me if I've lost weight." As if that is even possible cause don't forget who it is I'm talking to here; my PERSONAL trainer who is extremely fit, like they all are in the first place!
So the bottom line answer is: The fact that black is slimming is universally agreed. The reason why is simple: It hides all the SHADOWS that are created by lumps and bumps because you don't get shadows with a colour that already is the shadow.
I would say a shadow upon looking at it always looks gray (compared to the colour–don't forget in the colour world context is everything) like it does in the above photo but technically a shadow is the darker version of the colour created by the shadow and since black is the darkest colour, well that's the reason why it's the most slimming colour.