These appear to be reprints of the original material from the late 30s to the early 50s. I believe the books are The Black Bat's Triumph, The Black Bat And The Trojan Horse and The Black Bat's Dragon Trail, based on Google Translate's rough translations of the titles. Those came out consecutively in the early 40s.
So Blitz is among the chronological original material reprinting scene and they are using updated versions of covers from the first German series for their current releases. Example below.
The artist for those covers, by the way, was the extremely prolific Rudolf Sieber-Lonati. He was responsible for a ridiculous amount of cover art for crime and fantasy books from the 50s to the 80s but I believe he was best known for his science fiction work.
In any event, researching him specifically led me to a boatload of Fledermaus covers I still had not found, though they are of small size and/or of poor quality. I might have a little work to do there but the collection is becoming quite complete. Here are a couple that struck me as being particularly eye-grabbing.