Any way to adjust black level on RNG110 cable box?

I would say no, at least at the customer level. For that matter, it might be a hardware limitation. Who makes this "RNG110"?

16-235 is the standard range for broadcast video levels, 64 to 940 in 10 bits. 0-255 is a computer standard. In production systems video is sometimes full range but even at that it 4-1019. The first four and last bits are reserved for control words analogous to analog video sync pulses.

There are consumer video processors like the Lumagen and DVDO that can convert broadcast video to full range but it's just a numbers trick. You can't add back dynamic range that is not there to start with.

Well a scaler can fix what you don't like. It can map the 16-235 into 0-255. It just wont add missing levels. It works by something like "every 10th byte is used twice". Now there may be some higher end scalers that do a dynamic remap and better SIMULATE the missing data but they can never accurately replace it. Make sure the device can do this on an input by input basis or I guess you could use a preset.

Look at Lumagen and DVDO. You can find used units on EBAY. Also if you want to pursue this ask in the "Video Processors" forum. Someone has probably gone down this road already.