Ah, I see the confusion. You're mixing up "developer" with "publisher."
Blizzard Entertainment was owned by Vivendi Universal as part of its Vivendi Games umbrella. Vivendi Games and Activision merged (with Vivendi given a controlling share), rebranding the new publishing entity as Activision-Blizzard (since Blizzard was far and away the biggest name in the Vivendi side of things). Activision-Blizzard is a publisher, and has many different development studios: Infinity Ward, Treyarch, Red Octane, et cetera, just like EA has Harmonix, BioWare, and Mythic, or Sony has Naughty Dog, Team ICO, and Santa Monica.
All of these studios are separate entities that make their own games; the only thing they have in common is that their games are funded and published by the same corporation. Blizzard is actually a special case in that it self-publishes its own games (look at the box for Wrath of the Lich King, you won't see Activision's logo anywhere on there), since it's kind of off doing its own thing.
But even within the same publishing umbrella, different development studios are still different development studios.