Wirepuller
Mastermind, Black Curtain
noun, の adjective
Just imagine a black curtain off in the background. You don't know what's behind it, but you know what's in front of it. Thing is, the thing in front of the curtain is controlled by whoever is behind the black curtain. This person is the wirepuller or mastermind of the situation. Like the Wizard of Oz, or something.
The reading is the vocab readings for 黒 and 幕, though the vocab reading for 幕 is the same as the on'yomi reading, making things easy for you.
結局、あんたが黒幕だったんだな。どうりであの女を殺した時手慣れていたと思ったよ。
So, you are the mastermind, huh? No wonder you seemed so professional when you killed her.