When adding a new wallpaper in the Photos app, the photo is zoomed in automatically. These black borders account for this so that you can immediately hit the "Set" button to set the wallpaper without having to do any adjustments to the size or placement of the photo yourself.

The free version of Notcho adds these watermarks automatically. If you would like to remove them, you can purchase the Remove Watermark in-app purchase by tapping the "Remove Watermark" button in the Options menu. Once purchased, you must re-save the wallpapers again to create new ones without the watermark.