Black puddings have this rather unique split power:
Slashing and piercing weapons deal no damage to a black pudding. Instead the creature splits into two identical puddings, each with half of the original’s current hit points (round down). A pudding with 10 hit points or less cannot be further split and dies if reduced to 0 hit points.
When a pudding with 10 HP is hit with a slashing or piercing weapon, what does happen?
Deal no damage. Says right in the tin, Slashing and piercing weapons deal no damage to a black pudding. it is a very small black pudding, but still is a black pudding. Won't split.
Deal damage as normal. Since it cannot split, it takes damage from slashing and piercing weapons now.