Ditto Ben N, but let me add a couple of points that are too long to fit as comments.
I'd emphasize the distinction between layered and un-layered data formats. Drawing a black box over a section of a GIF, JPG, or PNG image destroys the previous contents. Drawing a black box over a section of a Photoshop, Corel Draw, or Paint Shop Pro native image does not destroy the previous contents if it's on a different layer.
I'd be very cautious about blurring. You'd have to know how the software does the blur. If the blurring does not involve any randomness, if it's a deterministic algorithm, it may be possible to undo the blur with appropriate software. No way would I rely on it without thoroughly understanding the algorithm. Unless there was some very good reason to blur rather than black out, I just wouldn't do it.
Of course any attempt to redact with solid blocks must completely cover the original contents to be safe. You want to draw a black box, not scribble over it with a black pen that might leave gaps.
Some formats may keep an internal history log. Not quite the same thing, but I once had a case where my organization produced documents in PDF, another company edited those documents and then sent then back to us. We found that errors had been introduced in the documents and, to put it bluntly, blamed them. They claimed that the documents must have been like this to begin with because they didn't do it. Apparently they were unaware that PDF has an internal log of all changes, and I was able to identify exactly what text was changed and the exact time and date of every change.