Section Background Video Flashes Black when Loading

Hello all!

This is my first forum post so please let me know if I leave out any information. I'm working on this website for a client:

http://goose-pentagon-x49w.squarespace.com

(It's public.)

The client wants the video of the gray brain scan in the background of each header to play automatically, but I am having a hard time getting rid of a black flash that occurs when the page is loading. I added a white mobile fallback image, and I've tried some code to change the background or hide the video until it loads, but nothing has worked thus far. 

Any help is greatly appreciated!

It looks fine to me, it shows white, not black.