Text stories in comics could generally be broken down into several categories. Some characters only appeared in text stories and were generally of the same genres of pulp fiction of the day: adventure, western, detective, etc. Centaur often had several of their ongoing adventure characters who had their own strips appear in text stories and some of them have some wonderful art by great Fred Guardineer. Sci-fi hero Dan Hastings was one of these and whose adventures in text and strip would span comic companies. Then you had the superheroes who appeared in their own strips but also text stories. Centaur's own Amazing Man was one of those. Harvey had their stories-behind-the-covers that told of the amazing adventure that was on the cover but not in the book otherwise.
It was odd to see a masked hero appear only in an text story and only in that one. Yet, as far as I've been able to ascertain, we do have two such cases, once with Mystery Man and here with Dr. Darkness. I've yet to come across other stories with either character. It's possible other stories were planned but just never made it into print. Neither is a bad little story.
Dr. Darkness appeared in 1940 in Keen Detective Funnies #24. More of the comic can be seen at www.goldenagecomics.co.uk