My employer makes a tool that does this! It's a screenshot tool we had been using internally for years that we released last year. It takes a sample of the colors in the image and uses them for random-looking obfuscation. So it's obvious that the data is hidden, but there's no pattern or way to un-blur that data. It doesn't work as well on low-contrast images, but we find it quite useful for hiding sensitive data. Some examples:
frog: https://cdn.zappy.app/f97505ba92625a0e949aebcfa4220852.png
wikipedia: https://cdn.zappy.app/3b7d5cf750066633e40aeddda926f95f.png
It's not really targeted at the HN crowd (requires an account, etc), but the app is "Zappy" if folks are interested.