To cut off the flow from the black tank, there is a handle pull in your UDC (universal docking center). There will be a stack of 3 handle pulls. Generally, the bottom handle is marked Black Tank.
If however, you're asking where the actual valve is for the black tank, generally, it is on the off-door-side of the coach on the end of the tank. There is usually small section of pipe on the tank output that is then connected to the valve. The valve is actuated by the cable pull.
Does this help? If not, please be more specific as to what you are trying to do.