With the normal or Add to Sample color dropper, point and click right smack-dab in the middle of a medium blue area of the image. The grayscale image in the dialog box will change to reflect the color selection. The white or light areas represent selected or partially selected pixels, and the black areas are not selected, or masked out. One click gets some of the blue, but it is clear how imperfect the bluescreen is, as only the upper half is selected with this first click... No worries!