Karen Swallow Prior at Marginalia Review of Books:
It is perhaps partially owing to this plain style that Black Beauty is now classified as children’s literature. (But it is perhaps owing, too, to a modern day disdain for simple things like manual labor, animals, imagination, and kindness.) During the mid-Victorian era, children’s literature was a still-developing category of books. The fact is that Sewell was writing for men. And she was read by them, too, as one review attests: “Both men and boys read it with the greatest avidity, and many declare it to be ‘the best book in the world’.”
While horse stories and talking animals are relegated today to children (especially girls), literary categories were less rigid then. So although the premise—a tale narrated by a horse—might seem childish to modern day readers, such a conceit was a novelty, edgy even, in its day.