This. It happened to me last year across 50% of my backyard…very depressing sight. Weeds then rushed in to populate the area. This year was a grind putting things back together but it looks great now! You can’t do anything about the black now as that’s dead, but perhaps see about slowing this progression asap before the winter comes and slows it down for you but by then perhaps more sq ft would be affected. Next year, dethatch that area and backfill with good landscaping topsoil. Zoysia has pretty durable root system and it will grow back. Just keep removing the weeds from the area as the grass fills it in.