Photomontage > 5.6 - Using Photoshop Elements > Layer Masks
That's because the white in the mask reveals the blue layer. 5 – Active Mask or Not Active Mask. The layer mask has a cyan rectangle around it. That means it's active. Whatever happens next—will happen to the mask. The blue layer is no longer active. If you double click the blue layer—it now has a cyan rectangle around it. It becomes active.