Video production
The videos are so cool!
I worked as a videographer for several years, and your problems in videotaping the black agitators comes down to a question of contrast. There are a couple of things you can do to try to help with this.
First, use very flat, diffused lighting. Bouncing a bright light off of a large piece of Foam-Core down towards the opening would work well for this.
If your camera's lens can accept accessories, it is possible that a polarization filter will help some (this will certainly help minimize glare on still water, probably not so much for the suds)
If you can get to some real video controls in your editing software, look for something named "black level" or "Black Level Setup" or something along those lines, and turn that up a bit. This will bring the black out of the shadows a little bit. Then, you'll need to turn down the "Luminance" or "Video Level" controls to bring the whites back down out of the stratosphere. If the controls are more basic, try turning the "contrast" level down and compensating by turning the "Brightness" level up.