That has to be one of the worst xkcd comics I've seen in a long time. The left picture shows a white and gold dress against a blue backdrop, and the right picture shows a blue and gold dress against a yellow backdrop. In neither picture does the "gold" look remotely like anything that could be called black.
I think a lot of this confusion is coming from the fact that the white balance of the picture is such that the blue fabric looks like evening light scattering off a white surface (a very light blue), so our eyes are interpreting that as the "white" point, and correcting everything else in the picture to match. So we have a way overexposed very dark object made to look like a slightly underexposed light object.