When a video is embedded into a page, it displays as an empty black box. The only way to avoid this is to manually upload an image and add as a Featured Image, which then displays correctly as a placeholder.
Ideally this should happen automatically, taking the first frame of the video, or one from a few seconds in. Even better would be a choice, the way that YouTube etc works.
Thank you for your suggestion. We will surely consider this in our next team meeting and users are encouraged to upvote this thread if they feel like this will be helpful. If it receives multiple upvotes, we will prioritize this feature in our next update.