I'm not sure if this might be a device diference, but when I take a screenshot of, for example, a Chrome incognito tab, my device shows "This app does not allow to take screenshots" and does not take a picture, instead of producing a black image as you described.
I know this can be set during runtime, because you're allowed to take pictures of normal tabs and you can also tweak the app's settings to allow you to take screenshots in incognito.