*IMPORTANT UPDATE*
It has been brought to my attention that my original calculations on the videos were
wrong. I have narrowed it down to a known issue with VLC where the "next frame" feature causes either freezes or jumps in the video (and sometimes visual corruption). In short, it's not reliable.
I redid calculations using WMP (using Ctrl+click on play button), and here's the new results:
Hollow Knight jump - 7 frames delay (116.66ms)
Hollow Knight attack - 7 frames delay (116.66ms)
Blossom Tales shield - 4 frames delay (66.66ms)
Blossom Tales sword - 4 frames delay (66.66ms)
Wildly different, but still, Hollow Knight still has almost double the input lag of Blossom Tales.
I have also updated the OP with these findings. Feel free to raise concerns if you have any. I wish someone with actual professional lag-measuring hardware would chime in. Digital Foundry?