I'm in a similar situation and probably with the exact same problem.
I'm using a ledwall that when the sending card si connected to my (or another) computer directly the blacks are displayed correctly but when I get the signal from a Teranex Mini Optical to HDMI 12G the blacks are displayed as grey.
I think this is the same problem of YUV<->RGB incompatibilities. I'm sending an RGB signal because my sending card likes it, on the documentation of the Teranex it says that it handles both RGB and YUV maybe with loading the proper LUT that I can't find anywhere.
Can you help us to fix this problem asap?