Anna Sewell's choice to make Black Beauty the central narrator of her story, and in fact to tell a horse's life story at all, is what made Black Beauty so groundbreaking at the time of its release. Beauty often points out that as a horse, he can't speak up for himself: "[…] If I could have spoken, I could have told my master where his oats went to" (30.10), he says when he witnesses a thief stealing his dinner. But in this book, he sure can talk.
He's candid, engaging, and observant. Do you want the dirt on the stable boys, or the scoop on the situation up at the manor house? Beauty probably knows. Not only that, he's an incredibly believable character. You may find yourself amazed that he wasn't a real horse. Alas, Anna Sewell did not talk to horses in order to write this book, though it often seems like she must have.
By using Beauty's point of view, Anna Sewell makes readers feel empathy for working horses, pointing out the ways in which horses are at the mercy of the humans who care for them. Black Beauty's narrative technique is probably its single most defining feature.