If I were to run cb on black kyurem, I would use freeze shock on the last slot simply because it has no other viable option except sleep talk (don't even say ice beam or hp fire). Its uses are extremely situational, but if you do land it, it will hurt like crazy. Predict a ferro switching in, hope it doesn't have protect, and let loose. If ferro switches out, whatever comes in will probably die or take a truckload. If ferro stays in, it will die. Theorymon also provided a stall breaking example yesterday in a pinch situation.
So yeah don't completely rule out freeze shock. It's not a good move, but it's not unviable.