Hi there 
I have a similar setup on my site and everything works perfectly. How are the videos stored in your database? Are you connecting the content via the Editor or via code? What video player are you using? You can only populate Wix videos from the database by using the single video player ( not the Wix Video app). The videos always show up as black initially for me, but work once you click play. If you can provide more info and maybe some screenshots I would be able to help you out.