What color do you see this dress?

[Losty]

I think this has to do with how the light is in the background and in the foreground.
The background is very bright.
The foreground looks like it's putting a yellowish hue on the black stripes of the dress.
Our brain is expecting lower light in the foreground so it interprets the blue as something lighter... White with blue hues.

But if your brain assumes homogeneous lighting in that room, then it darkens the color going through the eyes, and we get dark blue.

There's a TED talk on that:

What black stripes?

:p