The Poison Blade is an upgrade built by Leonardo da Vinci for the Hidden Blade which allows you to inject a small amount of poison into a person. For a short period of time the victim will wave their arms/weapon wildly before laying down and dieing. If one is too close to the subject they will be knocked to the ground and/or injured.
The Poison Blade can also be used as a distraction technique, as a poisoned person will gather a crowd. This can be useful in such missions as an Assassination Contract to distract guards away from a target.
Poison Bottles can be bought from the Doctor for a sum of 175f each, and a maximum of 15 can be held at any one time, providing you have all Poison Pouch upgrades.
Watch the Poison Blade in action.