Since it's android issue only, I'm wondering if you are you using a depth mask material/shader anywhere in your scene?  This is a common issue on some android devices.  If so you'll need to edit the shader "Ztest always".



If not, I'd start by seeing if a blank scene or the Vuforia sample scenes exhibit the same behavior with your Unity/Vuforia install.

 

I'm seeing this but ONLY on Android and NOT on iOS! Also, this wasn't happening for me until I attempted to update to Unity 2019.1. This required an update to Vuforia. There were major issues with the upgrade including my AR scenes taking 10 seconds longer to load than previously (all UI locked up during this period). I then rolled back to Unity 2018.3 with the AR loading issue fixed but the black flashes suddenly appearing on Android. I've attempted to roll Vuforia back to the version shipped with 2018.3 but uncertain if it's having effect. It's my priority to get this fixed so I'll be looking into it heavily for the next few days. If I turn anything up I'll post back here...

That is the first thing I checked, but unfortunately it isn't that. It just affects the Video-Background and depending on which angle it is more or less intense. I am starting to think it is some kind of bug. I have changed the 3D content, deleted all the UI elements, have been trying a thousand things and nothing.

This is usually related to the near clipping plane on your camera.  Adjust the numbers and see if it helps.