Blur Operation on a Photo : App Recommendation? macOS Preferred; iOS OK

If you wished to do a simple blur operation on a photo, what tool or application (app) would you use for speed an efficiency?

I use Annotable on iOS for marking up photos. It has a couple of blur options.

For macOS:

I would use Acorn.
If money is tight, GIMP is free.
If this is something you want to batch from the command line, I would use ImageMagick.