I don't quite understand the phrase "More than a million boys’ periodicals were sold a week" - it seems to mean "More than a million copies of the penny dreadfuls (periodicals) subscribed by boys were sold a week." I am not sure at all.
What does the phrase mean?
The penny dreadfuls were printed on cheap wood pulp paper and were aimed at young working class men. More than a million boys’ periodicals were sold a week, but the popularity of penny dreadfuls was challenged in the 1890s by the rise of competing literature, especially the half-penny periodicals published by Alfred Harmsworth.
Your interpretation looks correct. I think the writer wanted to avoid re-using the phrase "penny dreadful" in this sentence, and perhaps wanted to include other less "dreadful" forms of literature. However, shortly before, the writer tell us that "penny dreadfuls were aimed at young working class men." so it reasonable to assume that "boys' periodicals" is another phrase that means "penny dreadfuls."