Set Black Wallpaper on Android devices with OLED screen helps to get more usage with the battery, Setting a black wallpaper is like removing the current wallpaper and as if you are using No wallpaper.
Try one of these SIMPLE ways:
1. Download a No Wallpaper app from Google Play. since these types of apps are actually a live wallpaper, they still use the RAM to run and is not recommended
2. Open the camera application, put it on a surface facing ground, and take picture. Now use this dark image as the background (Or you may just cover the lens with your finger in a dark place)
3. If you have a Picture Editor installed on your device, try cropping a dark part of an image, and use it as the background
4. Open Photo Editor on Galaxy devices, select an image to edit, select Decoration following Pen, Choose the color (dark), and now start brushing to fully cover the image with selected color