I have swithed on the “Blank out other screens” in Compose tab of Preferences window and expected the second screen gets dark after entering the Composition mode. But it doesn’t. Regardless of the Full screen mode of Scrivener.
As far as I remember this feature got unusable after upgrading to Maverics (now I run Yosemite). I was also expecting the 2.7 upgrade will solve this problem, but it didn’t.
Here is the latest word on that.