Not specifically Monzo or banking related, but because people often post screenshots and other things to this forum with personal information redacted… consider this a PSA 
The above is an interesting tech demo and great explanation. (Video is concise and comprehensible, and text has further details.)
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So this isn’t safe then?
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Best ROT13 it, to be on the safe side. Or ROT26, twice as secure!
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I saw something about when people black out text, that if you change the brightness/saturation back and forth, you can view what’s under it.
I’ve tried it 3 or 4 times with pics from different people and I’ve never got it work.
edit
I missed out the most important word here. I’ve never got it to work! 
Think that depends on what editing program you’re using, or not looking closely at the default settings. A 100% opaque black painted mark, or area fill, wouldn’t reveal anything. If it’s anything other than 100% opacity, or you’re using the highlighter tool… yeah, your trick will work.
If you’re having to ‘scrub’ backwards and forwards to black out text, there’s an opacity issue.
tl;dr, use a pen or shape tool - not a brush or highlighter tool - when blacking out text.
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What’s my secret then?
As @HoldenCarver says, I think that’s only true if someone simply layers another vector graphic on top of the actual text. If you’re using a bitmap editor and exporting as a bitmap this shouldn’t work. There’s also a specific ‘redact’ feature in MacOS Preview (and I assume other PDF editors?) that I believe obliterates the underlying text instead of just covering it up with another object. This issue is actually mentioned towards the bottom of the article linked 
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Haha, I missed out “never” from my post!
I’ve tried it a few times and I’ve never got it to work!
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No, I don’t think that would even fit into the length of the redaction!