Not sure I agree with that.
Here is what happens:
YCbCr mode only outputs limited range. It does not support full range. So your card sends 16-235 and monitor receives 16 as minimum and 235 as maximum value. Things look as they should
Now if you switch the card to RGB full mode but leave the TV in HDMI Black level Auto/Low:
Card sends 0-255. TV receives it but treats 16 and below as pure black and 235 and above as pure color. So in effect you crush the blacks and whites and over saturate the rest of the image (colors get brighter).
When you switch to HDMI Black level normal
Card sends 0-255. TV receives and expects 0 to be minimum and 255 to be maximum brightness. Things look as they should.
If you do not believe me do this test. Set contrast to 100. Brightness to 45 and back-light to 20.
Set the card to output RGB Full mode. Set the TV to HDMI Black level auto/low.
Open this image:
http://www.lagom.nl/lcd-test/black.php
See how all the blacks got crushed. Boxes 1-15 now all look black.
Ok now increase brightness to 60. Now the blacks look correct again.
Then keeping HDMI level low and 60 brightness Open this image:
http://www.lagom.nl/lcd-test/contrast.php
See how you cannot tell the boxes 29, 30, 31 apart anymore ?. They are all max brightness.
Now lower contrast to 90 in TV. Now you can tell them apart. You are basically correcting here for the TV interpreting colors below 16 as black and colors above 235 as pure color by increasing brightness and lowering contrast.
Now keep the HDMI black level low. Go back to 100 contrast, 45 brightness but go to your control panel and have the card output RGB limited. The above two images will look correct. No white crush no black crush.
Now go back to HDMI black level normal and RGB full. Things will look normal still.