In the long, long ago, before Photoshop…
You muted the background of a photograph by painting something slightly opaque on the surface of the picture itself, as you can see more clearly in the bottom photo. You could also indicate the area of the photo you want used by actually scratching the boundaries into the photo paper.
Caption: “Electronic organ recently used to entertain audience of moving picture theater. Music is produced by thyratron electron tubes.” Schenectady’s WGY was there to witness this momentous occasion.
From the Library of American Broadcasting collection.