A black background for your Android home screen is a great way to improve the battery life of your phone. The same idea applies to your lock screen. When you disable the current wallpaper, set the black one instead. Then you can change the wallpaper back to its previous setting if you want. Just remember that you must uninstall the old one first, if you want to make the new one last longer. Then, you need to restart your phone to apply the new settings.