Written in 1877, Anna Sewell’s classic children’s novel “Black Beauty” warned against the abuse of horses. The pristine adaptation streaming on Disney+ is a melodrama in need of rougher edges. Directed by Ashley Avis (a former competitive equestrian), the movie is set in the present-day United States and features two female protagonists: the fiery mustang Black Beauty (voiced by Kate Winslet) and the recently orphaned Jo (Mackenzie Foy), now living with her uncle John (Iain Glen), training horses at Birtwick Stables.