Legal broadcast colors and graphic color space are two different things and trying to please two masters… well you know how that goes.
I would suggest you correct all the blacks in the program to be correct, this way a DVD on television is correct. This is black at 16:16:16 (8 bit RGB) and white at 235:235:235 (8 bit RGB).
If you want to create a version for computer viewing, select RGB color in the Quicktime export which will stretch the 16-235 to 0-255. This is helpful for some MPEG2 encoders that expect that range as well as video streaming formats seen on computers. But in doing so, any black detail that was in the 0-16 range before doing this will be gone.
If you just want to have your black bars match, then create your own in PhotoShop with 0:0:0 or use the Title Tool to make the bars as they are 0:0:0.
I don’t know what might be causing the white blanking. Is it over the entire program?
Michael