Very beginning of the game, before Edward even found Assassin's clothes. If you decide to go hunt a wild boar, Edward takes out his swords, but when he approaches the boar, swords get steathed magically and instantly, and he kills the boar with hidden blade that he doesn't have yet!
There's no hidden blade in inventory, but that doesn't prevent Edward from using it. Respective animation and sound play, and wild boar actually dies.
...
https://www.youtube.com/watch?v=bk7p964lnEE