The game engine, no. But the game logic, the code that makes the game engine do the things that make what it's doing Halo as opposed to some-other-game-on-that-engine, is written in blamscript.
"Screenwriting" to me would sort-of imply that this was all just for human consumption during planning, and later implemented in something else.
I first meant to use its synonym, "scriptwriting", but that one of course has a somewhat diverged meaning in the computer world. But actually in interactive fiction the meanings beautifully converge again.
