Her eyes are not really blue! There are no blue pixels in this image. See proof below.
Her eyes are grey, not blue. You can test this for yourself in an image editing program, or see the illustration below.
The illustration shows a constant grey from one circle to another. After observing this, both eyes may look grey, but then focusing on the left side of the image may cause the blue color to appear again.
This illusion takes advantage of color constancy, which is the ability of our brains to make color corrections to what we see. This allows us to perceive colors even when they are in shadow or in colored light. In this case, the red filter covering all of the image except the eyes tricks our brains into “correcting” the grey eyes to blue.
If you enjoy optical illusions of this sort, I highly recommend the book Masters of Deception, which highlights the work of 20 different artists who are experts in visual trickery. The book contains some of the best work of well-known artists like M.C. Escher and Salvador Dalí, as well as images by Akiyoshi Kitaoka, the modern master of color illusions.