Hi All
Davinci Resolve 16.1.1.005 free version
i've been having similar issues when using blanking fill on stills and video
Essentially I was applying BF from the timeline page on an unaltered image/video and all was well, however when any transformation was applied, in my case I was performing an image flip, it stopped working, showing only a black area around the clip.
I have poked around on various forums and found some suggesting that appling BF from the colour edit page solved the problem, however it was not working for me...until
I have found that transformation needs to applied from the colours page
on the colours page select SIZING panel then NODE SIZING the clip can now be adjusted
see image below for the panel in question
also it seems this will work whether the BF has been applied from the edit page or colours page

- Screen Shot 11-02-19 at 11.11 AM.JPG (53.56 KiB) Viewed 7418 times