Hi Koh, welcome to MobileRead!
What you're describing as a "black screen when turning pages" has the "scientific" name of ghosting. It is in a certain way inherent in the eink technology. The Pocketbook offers different settings you can choose from from doing a page refresh every time you turn a page to doing it only every 5 or 10 pages or never. While the first options leads to almost no ghosting but takes some time the last one has a lot of ghosting but does turn the pages much more quickly. So, to a certain degree you can deactivate the afterimage.