From wikipedia: The dress itself was confirmed as a royal blue "Lace Bodycon Dress" from the retailer Roman Originals,[26] which was actually black and blue in

From wikipedia: The dress itself was confirmed as a royal blue "Lace Bodycon Dress" from the retailer Roman Originals,[26] which was actually black and blue in colour;[1][2] although available in three other colours (red, pink, and ivory, each with black lace), a white and gold version was not available at the time. So objectively it is black and blue. Of course, our eyes tricking us about color is hardly anything new... (brown is just orange with context!)