FFmpeg: How to convert vertical video with black sides, to video 16:9, with blurred background sides

I solved!

ffmpeg -i input.mp4 -lavfi '[0:v]scale=ih*16/9:-1,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1[bg];[bg][0:v]overlay=(W-w)/2:(H-h)/2,crop=h=iw*9/16' -vb 800K output.webm

Input: https://www.youtube.com/watch?v=17uHCHfgs60
Output: http://www.youtube.com/watch?v=CgZsDLfzrTs

The accepted answer here takes forever to execute because it is doing so much unnecessary computation. We don't need to blur the pixels which we definitely know that will be out of viewport in the output video.

So, a better solution would be to first crop the part of the video which will be visible in the output. We then scale this part to "fill" the viewport. Finally, we overlay the original video on top of it.

Below example assumes that input video has greater
aspect ratio than output video.
                    ┌─────────────┐
┌─────────────┐     │             │
│ Input video │     │   Output    │
│             │     │   video     │
└─────────────┘     │             │
                    │             │
                    └─────────────┘

We will use filter graph to achieve this. Our filter will like below in dot notation:

                [original]
 input --> split -------------------------------> overlay --> output
        │                                          ^
        │[copy]                           [blurred]│
        └──────> crop ──> scale ──> gblur ─────────┘

Assuming the resolution for input video is 1280 x 720, the command looks like below:

ffmpeg -i input.mp4 -vf 'split [original][copy]; [copy] crop=ih*9/16:ih:iw/2-ow/2:0, scale=1280:2282, gblur=sigma=20[blurred]; [blurred][original]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2' output.mp4

I couldn't get either of the previous solutions provided to work using ffmpeg 3.4.2 on Windows.

However this did work:

ffmpeg -i <input_file> -filter_complex "[0:v]scale=ih*16/9:-1,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1[bg];[bg][0:v]overlay=(W-w)/2:(H-h)/2,crop=h=iw*9/16" <output_file>

Don't forget to replace <input_file> and <output_file> with the appropriate file names.