I believe you are exactly right on that being the effect of a limited RGB range. I have noticed this too, except in my case it was the camera's jpeg snapshot URL delivering a reduced-contrast Limited RGB image, and
Blue Iris displaying Full RGB.
In any case, Blue Iris does not have controls for Limited vs Full contrast, and I am not aware of any camera that has such an option either. Perhaps if you ask Blue Iris support, he might be able to implement a per-camera toggle that enables Limited-to-Full contrast conversion for a camera. Hopefully such a conversion would not incur a significant CPU performance cost.