It was hard to tell if there was any 'opacity' at all. Looks like a bright light over a dim background which seems like the only way you could physically do it, given what we know about their technology.
Magic Leap has a patent which includes: "occlusion mask device comprises a display configured to either occlude or pass light at each of a plurality of portions of the display, depending upon a pertinent command to occlude or pass light at each portion."