Change Sierra Login Screen Background Blur Amount
The image file is stored in Macintosh-HD:Library:Caches:

There will only be like 3 files there so it should be easy to find.

You can take this file and edit it in an image editor such as GIMP(free) or Photoshop(not free) etc.

Don't forget to make sure that the image keeps it's name or OSX will get confused.

You may also need to use the original image, if, you want to blur it down.

Hope this helps!