I’ve seen even this fail once, although it was unusual circumstances. Black box on what turned out to be slightly off-black text using a PDF highlighting tool. Not the ideal way to do it. When printed and scanned you could just barely read it if you had a monitor with a bad viewing angle or adjusted the contrast. A paralegal caught it before it went out.
Black boxes also fail when they leave out the bottom or top of characters.
I took down a ton of poorly redacted black boxes while modding /r/facepalm
In retrospect, a bot could do the job pretty well
Caught by pair a eagle eyes, eh?
You can also screenshot as an interim step, though not quite as secure as this. :-)