Comment Too much information is lost in the photo (Score 1) 420
Sampling the pixels directly from the image reveals that the dress color in the photo has a hue between in or around the range of 230 or so, which happens to be blue. However, because it appears that the dress was not directly lit, that hue may be arising because of diffuse interreflections with its surroundings, something that anything which has a lighter shade can be very susceptible to if the only light hitting it is diffused, and which is the kind of lighting that this dress does appear to be exposed to in the photo.
So there is simply too much information about the surrounding lighting conditions that has been lost from the photo to ascertain with any certainty what the actual color of the dress is... at least from this one photograph alone.
Debating the matter is pointless, because it is impossible to actually arrive at a logically valid conclusion merely from what one can see in this photograph except through sheer guesswork.