I believe it operates on a site-by-site basis since there's settings to tell it to ignore screenshots of certain webpages (at least they're doing that). So one window is blacked out but the other is visible.

Just gotta remember to set up all the sites you want blocked which I expect to be made as complicated as possible and a little nag window on the bottom left every time I visit a blocked site, i.e., "Recall isn't able to take a snapshot of this webpage" with a big "FIX NOW" button.

its the sites that produce some javascript bloatwarae that tells the browser to display a blank page when a screenshot/screencapture software is used.

in my opionion that is giving the javascript ecosystem way too much power, though that is life with DRMs ... pretty sad times when folks pay providers and allow the providers to disable a screenshot lol

It'll just block out the DRMed content and not the rest of the screen.