When you need to hide sensitive information in a screenshot or document, you have three main options: blur it, pixelate it, or black it out. They all look like they do the same job. But do they?
The answer depends on two things: what format you're working with (image vs. PDF) and whether the underlying data is actually removed or just visually hidden. Let's break down each method.
Blurring applies a Gaussian blur filter that smears the pixels in a selected area, making text unreadable. It's the most common method you'll see in tutorials and privacy tools.
The blur algorithm averages the color values of neighboring pixels, effectively destroying the sharp edges that make text readable. The stronger the blur radius, the more the original information is lost.
Bottom line: Use a strong blur radius, especially for short strings like numbers or codes. For multi-word text, blur is very effective.
Pixelation divides the selected area into a grid of large colored blocks, replacing the fine detail with chunky squares. It's the classic "censored" look you see on TV.
The algorithm splits the area into blocks (e.g. 10x10 pixels) and replaces each block with a single average color. The result is a mosaic pattern that obscures the original content.
Bottom line: Pixelation is less secure than blur for hiding text. If you use it, make sure the block size is large enough that individual characters are completely unrecognizable.
The simplest approach: cover the sensitive area with a solid black (or colored) rectangle. Nothing visible underneath. Problem solved — or is it?
When you draw a black rectangle on an image and save it as a flat PNG or JPG, the pixels underneath are permanently replaced. The original data is gone. This is simple and effective.
Here's the critical distinction that has caused countless data breaches, including the infamous Epstein documents failure:
A black rectangle drawn on a PDF is usually just a visual overlay. The text underneath is still there in the file structure. Anyone can:
This happens because PDF files separate content from visual presentation. Drawing a shape on top of text doesn't delete the text — it just covers it up, like placing a sticky note over a printed page.
Bottom line: On images, black bars work fine. On PDFs, they're dangerous unless you use true redaction that removes the underlying content from the file structure.
Regardless of which visual method you choose, the only thing that truly matters is: is the original data still in the file?
When you apply any redaction method (blur, pixelate, or black out) and export the result as a flat image file, the original pixels are permanently replaced. The data is gone. All three methods are permanent in this context.
The difference is only in how much information leaks through the filter:
This is where the stakes are much higher. PDFs are structured documents, not flat images. Visual changes (drawing shapes, adding highlights) don't modify the text data underneath.
True PDF redaction must:
If your tool only does step 1 and 3 without step 2, your "redacted" PDF still contains all the sensitive data.
| Method | Images | PDFs | Reversible? |
|---|---|---|---|
| Blur | Secure (strong radius) | N/A — needs true redaction | No (if strong enough) |
| Pixelate | Moderate risk | N/A — needs true redaction | Partially (ML attacks) |
| Black out | Secure (flat export) | DANGEROUS as overlay only | Yes on PDFs, No on images |
| True redaction | N/A | Secure (content removed) | No |
BlurData is a macOS app that handles both scenarios — images and PDFs — with the right approach for each:
For screenshots and images (JPG, PNG): BlurData automatically detects sensitive data — emails, names, addresses, monetary amounts, account numbers, license plates, IP addresses, and URLs — and applies a blur to hide them. The result is exported as a flat image where the original text data no longer exists.
For PDF documents: BlurData uses native PDF redaction that removes the actual content from the file structure. This isn't a visual overlay — the text is permanently deleted from the document. There's nothing to "unhide" because the data simply isn't in the file anymore.
The key advantage is that all of this happens automatically and entirely offline on your Mac. You don't need to manually identify every piece of sensitive data — the app detects it for you. And since nothing is uploaded to any server, your documents never leave your computer.
If you work with both screenshots and PDFs, having a tool that understands the difference between image redaction and PDF redaction is essential. The wrong approach for the wrong format is how data breaches happen.