This article will explain how the blackborder detection threshold works and will show a possible means to determine a correct value for the threshold.
The blackborder detection threshold value is a percentage indication. It describes the the percentage of 255 a pixel needs to have for either red, green or blue, in order to be treated as non-black.
So a threshold of 14% means that any pixel with a red, green or blue value bigger than 36 (255 * 14% = 35.7 ≈ 36) is treated as non-black. All other pixels are regarded as black.
A possible way to determine a correct black border threshold for your setup
is using a color grabber
and a screenshot from HyperCon.
Just take a screenshot from within HyperCon while processing a scene with black borders (you need to be connected via SSH in order to be able to take a screenshot).
Now use a color picker and grab the RGB value from a couple of locations within the black borders (for example gpick available for raspbian can do this job). The maximum red green or blue portion value of all picked pixels will be the basis for your black border detection threshold.
You just have to choose a percentage value that results a bit higher RGB portion value.
When color picking from the borders I saw values between (25, 25, 25) and (31, 32, 31) returned by my grabber
So my maximum percentage value was 32 / 255 ≈ 0,1255 ≈ 13%.
In this case any value from 14% on should be fine, in my case I selected 16% in order to be on the safe side.