You can book this trip at the tour desk at the hotel. Transportation is typically provided by the tour operator. However, just in case you are not aware, this would be an all-day excursion. To get to YS from Ocho would take at least 4 hours (YS is south of Negril). The cost would also be higher since the transportation cost would be a longer trip. I'm not sure you could do both Black River and YS Falls in only one day due to losing time just getting there.
If this is till something you want to do, go for it. But if you want to climb water falls, stick with Dunn's River, which is already included as part of your stay at CSS.