This is a sequel to A Take of Three Blurs, which I wrote in 2006 as a guide to blurring effects in Adobe After Effects.
Twelve years later, After Effects introduces some new blurring, some new best practices, and a new level of confusion.
Use Fast box blur now for everything.
But come on, there's so much more to know about After Effects blurring!
The three oldies
This was the breakdown in 2006:
Fast blur was a good, universal blur. It was "fast" because it was just a box blur with three iterations and box blurs on the CPU can be fast. Most users choose blur by default at this point.
Box blur has exactly the same blur engine as Fast Blur, but with control over the number of iterations. With three iterations, it fits perfectly with Fast Blur. An iteration creates a useful square blur effect. With more than three iterations, a smoother, rounder blur is created than with Fast Blur. My advice was to reach for this blur the most, as it offers the most control.
Gaussian blur also used the same blur engine as Fast Blur, also with three iterations. However, since there was no option to repeat edge pixels, my advice was never to use them.