To use the Color Changer you need to first sample the color you want to change by holding the Ctrl key and clicking ONCE only on the blue.
Don't click on anything else.
Then change the Foreground color in the Materials Palette to White and click in the same place again.

Have you tried adjusting the Tolerance and Edge Softness in the Color Changer tool?
Try it with different settings, starting with 0 in both.

You could use the Magic Wand to select the blue and then fill the selection with white.
Tolerance at 20 (the default setting) should work ok.

Grenou