A black screen appears, if there is not enough free space on your device to save the data. Below each downloadable content in the Klett Sprachen App you will see the size in MB in brackets. Make sure you have enough space and then restart the download.
Note: If you have finished working on a chapter, it is very easy to erase it from your device with the "change" function. Don’t worry, you can always download it again if you need it.