I've finally managed to open-source this project which I created at work, but which is now also in use by Mastodon:
https://github.com/woltapp/blurhash
There is a nice little demo page where you can play with it here:
Toot! already supports it for image placeholders while loading, but other clients should easily be able to include it too.
 
      Toot!.app
Toot!.app All Jonkman Microblog content and data are available under the
 All Jonkman Microblog content and data are available under the