With RGB Gain/Bias control it all depends on how they have been implemented in the specific TV's CMS.
Usually, Gain is high (whites) and Bias low (black).
And when adjusting Gain the rule of thumb is you reduce the value, not increase, or you will potentially cause clipping.
(And Green is usually not adjusted, other than to lower peak luma.)
When adjusting Bias the rule of thumb is you increase the value, not reduce, or you will potentially cause crushing.
But, with low-lights most displays have a colour tint in the blacks.
That is inherent with most LCD screens for example - a blue bias.
As that is inherent it can't be reduced near black.
This is all explained in the Manual Calibration Guide
Mob Boss at Light Illusion