Why does my downloaded slideshow have a black background?

When you download a video version of a slideshow, the video file will contain a black background. This is because the video may be viewed on a wide range of screens, which may not always have the same ratio as the video file. In these cases, the video is resized to fit the proportions of the screen and as a result, will produce black borders around the video during playback.

Keeping the background black ensures that there is a consistent background while viewing the slideshow on all screen sizes and is a universal standard for video files.