The link below is to a 10 by 10 pixel jpg to be used as a desktop background. Windows, OSX and iOS do not generally provide a desktop background that is black. I started using a black background on my iPhone to make sure all the pixels are switched off to save on battery life, it looks good so I use it on all my machines.
Here is a black image - it is a 10 by 10 JPG - click on it and download it to your Pictures folder and you will then be able to select it in the Wallpapers and Backgrounds dialogue.
