Layer Selecting a color For the following example I selected a color from the background of the landscape layer. If a color in the underling layers is lighter than the blend colors then the blend colors will be less have less opacity, i.e. more of the underlying colors will show through the blend layer. When the underlying colors are darker than the blend colors, the blend colors will have more opacity, i.e. less of the underlying colors will show through. That is why the trees, which are darker than the blend layer's blue color, do not show through the blend range. Notice how the colors, in the sky, that are nearer or the same lightness value as the blend range's blue color are subtracted leaving a a very dark near black value.
Lets take a look at another example.
In the above example the trees being darker in lightness than the Blend layer's color do not show through the Blend color. The sky colors of the landscape are closer in lightness to the Blend color and are beginning to show through the Blend color. Lets take a look. Blend color Blend results Notice how, in the above example, the trees are now beginning to show through the Blend range. Not only are some of the colors in the trees lighter than the Blend color, the colors in the sky and grass are lighter, so the Blend color over those lighter underlying colors have less opacity.