You can add a page with a black background, though not a single paragraph currently. To do this, you can create a blank page image with a black background and use it as a background image for the first page of the chapter. However, please note that this method only applies to the first page of each chapter, not all pages in the book.
You would also want to set your theme to use Individual Chapter Images, unless you want the first page of every chapter to be black. This tutorial may help: https://www.atticus.io/create-a-custom-theme/