Overwatch was Blizzard's first new intellectual property since StarCraft.
Blizzard unveiled Overwatch at BlizzCon in November 2014 and released it in 2016. The game arrived after years in which the company concentrated on established universes such as Warcraft, Diablo, and StarCraft, making a brand-new setting notable for the studio.
Overwatch was not created from nothing: much of its development team and some concepts came from Titan, a massively multiplayer project Blizzard cancelled in 2013. After Titan's cancellation, developers reshaped its ideas into a character-focused team shooter inspired partly by games such as Team Fortress 2 and multiplayer online battle arenas.
The wording can cause a minor historical mix-up. Some contemporary coverage called Overwatch Blizzard's first new franchise since Diablo, while other references describe it as the first new intellectual property since StarCraft. For this question's specific phrasing and the cited historical account, StarCraft is the canonical answer. Hearthstone was not a new Blizzard universe; it is set within Warcraft, while Warcraft and Diablo were already established franchises.