I have never heard of black water but I have heard of BACKWATER.....It's an older term used by old country vets, that used to mean the horse had fluid on the spinal column, And it could leave a horse with loss of use of its hind end,bowl movements,urination. Most horses are put down unless the injury was minor and would resolve with time and care. You can often see or feel where the pressure is at,and when you can't it just means the pressure is on the underside of the spine. Some vets will try to drain the fluid but until it heals( if its going to) the fluid just comes back.