What event in 2015 prompted Musk's public commentary directed at Blue Origin's progress?

When Blue Origin demonstrated a successful landing of its New Shepard booster.

The competitive dialogue between the leaders of SpaceX and Blue Origin is not limited to recent years; it has established a recurring pattern dating back to the early stages of reusable rocketry development. Specifically, following one of Blue Origin's early achievements in vertical flight testing, Musk offered public commentary directed at the outcome. The text pinpoints that when Blue Origin demonstrated a successful landing of its New Shepard booster, Musk reacted publicly to this event in 2015. These early exchanges established the precedent that one company's vertical flight test immediately becomes a subject for commentary by the principal leader of the other firm.