Black Jade Pool was a Spiritual Pool, something naturally formed of the heavens and the earth, that would provide the topmost nourishment for the body and spirit of cultivators. Moreover, after being taken over by the Scarlet Heaven Academy, a lot of precious treasures had been added into the pool to further enhance its effects.
However, this pool would only be effective for those of the Shattering Void Tier and Mountain River Tier. It would have no effect for those of higher levels, or else it wouldn't have been placed in the Scarlet Heaven Academy.