So, it turns out this "black flicker" thing is a problem that is widely reported elsewhere. Type the following into google
obs mac capture flicker
you will discover dozens of items in the search results that show this is a pervasive problem. The OBS developers do not seem to have found a solution, which has a lot of people grumpy.
But, I believe I have overcome this problem. I have sent a new version to Fred and am awaiting his feedback.