Hi,
normally the poster should be displayed until the browser serves the first video frame - but unfortunately the browsers are not always reporting their video state correctly. Sometimes there is indeed a black-frame for a short time (I think mostly on iOS and also depending on the video itself).
Just displaying the poster until clicked is unfortunately not enough, that could still cause black frames for a short time when the video starts to play.
But improving the current behavior is already on my list. One idea is to render the video into a small buffer and analyze its actual pixels and check if they are all empty or black... (although this can be problematic when video actually starts with a black image

)
Best regards,
Klaus