User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-41.69.147.157-20140806015457/@comment-13471981-20140806083731

Just because Julien du Casse collects Hidden Blades doesn't mean anything; he's probably the exception rather than the rule. Shay has a Hidden Blade because he was previously an Assassin and, since it's a very stealthy weapon, decided to keep it when he turned his back on the Order (or when the Order turned its back on him, I suppose).