I'm looking at the image on a very high quality OLED screen. On an OLED, it's very obviously NOT black. You can see the top of the dress in the light and it's not at all black. The black on my screen makes that incredibly obvious. Who cares what it is in real life, I'm not judging that, I'm judging the image and black is not a part of the dress, in this image.
The Guardian's image is black and blue. The image in this thread IS NOT. My eyes may look red in a photo, that doesn't mean they're really red. An image is not a perfect reflection of reality.