Ah jeez, Leonardo did not invent the hidden blade. Nobody knows who invented the original hidden blade, but the first use of it was by a Persian assassin named Darius (pic included). The original hidden blade required the loss of the left ring finger to be worn (why Darius has all 10 fingers was probably a concept art fuck-up) Altair was the one who invented the hidden blade that Ezio uses and what most assassins use. He used knowledge from a POE and he invented it so that assassins would not have to sacrifice a ring finger.