So the blu-rays were encoded RGB Full, rather than RGB limited?

As I understand it, TVs use a video range from 16-235. It considers levels below 16 to be black, and information above 235 is white. A calibrated TV will never display anything below 16 as anything other than black. Most will also treat everything over 235 as white since it should not exist in video content.

PCs are different and use a range from 0-255. There is no data below 0 or above 255 with an 8-bit video signal as there are only 256 possible values. In short, this is much simpler to understand as the TV concepts of Blacker-than-Black and Whiter-than-White do not exist.

Therefore to make the conversion between RGB Full (“PC Mode”) and RGB Limited (“TV Mode”) all you need is the following levels adjustment in AVISynth

// PC to TV ; scales a [0,255] clip to [16,235]:
Levels(0, 1, 255, 16, 235, coring=false)