I read a post which was posted in 2013 (https://developer.vuforia.com/forum/unity-3-extension-technical-discussion/video-playback-black-screen-appears-moment?sort=2) which shares the same issue, from which the resolution/workaround at the moment just delays the video from starting rather than removing the black square that appears at the start of a video.
As its been a while since that post was made, I was wondering if any further workarounds were found, whether these be regarding file size/formats or a different script to use.
Any help would be greatly appreciated.
Was tested on a iPad 3rd Gen and Samsung Galaxy S6. (Latest Unity/Vuforia installed)
EDIT: If of any use, the videos are stored in the StreamingAssets folder (played directly from device).