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.
What colour is a shadow, you ask?
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.