Go to the Layers panel and click on the upper layer to select it. Now click on the ‘Blend Ranges’ gearwheel at the top of the panel to display the Blend Options dialog. At the top of the Source Layer Ranges graph you’ll see a line joining a black node on the left to a white node on the right. It’s at the top of the graph because everything is at 100% opacity, but if you drag the right-hand end all the way down you’ll make the white pixels in your image 0% opaque (i.e. 100% transparent). At this point the black pixels will still be 100% opaque but everything else will be semitransparent; to restore full opacity to all the non-white pixels, drag the left-hand end of the line straight across towards the right, stopping when the white pixels begin to reappear.