The Black Arkansas stone will allow you to finalize your blade edge. This grade of a natural Arkansas stone is considered an extra fine stone and will allow you to put a mild polish on your blades. The Black Arkansas stone will allow you to complete a very fine edge on your blade. When compared to a man made stone the Black Arkansas stone is similar to 800-1200 grit. This is a pretty fine stone that will offer a final pass to give your blade a polished finish and a razor sharp edge.
I have discontinued using these stones for the primary ViperSharp kit. The nature of these natural stones is such that they cannot be made to the precise measurements needed. Since the stone thickness is vital for a professional sharpening system to function 100% these stones will be available only as an add-on and you should keep in mind that a digital angle finder or similar tool would be highly recommended to assure the same angle as the other man made stones.