We've encountered similar issues, random black shapes being cut out of the camera texture, with two possible triggers.
The first seems to be related to pose tracking (rotation/position tracking). On some of our test devices (Pixel, Pixel XL, Pixel 2 and Pixel 2 XL to be specific), having pose tracking enabled triggered the black flickering. If you uncheck 'Track Device Pose' in the Vuforia configuration, that seems to resolve this particular issue for us ... but of course, if you need that functionality, this isn't much help.
The second appears to be related to enabling and disabling Vuforia too rapidly during initialisation. In one scene, we had a Vuforia camera which was enabled in the scene, but disabled during 'start', and then re-enabled later, and this triggered black flashing. We resolved that issue by starting the scene with the camera disabled, and presumably allowing it to initialise properly when it finally is needed.
I hope one of these two is of help, but I've heard of other developers encountering the mysterious black flickering of doom, so I believe there are other triggers out there.