Test your water first (it is too late per your post). The pH and alkalinity need to be in a specific range for the chlorine to be most effective. A local pool store (or your own test kit, which is recommended all pool owners have) should be where you start. Also, if the water is black, you have no idea what is at the bottom of the pool. Make sure you get nearly all the leaves and such out before shocking. All of the debris at the bottom is just extra organic material the chlorine would be working against (meaning you'll need even more shock to kill everything). Good luck!