you will always loose some blackness in any conversion and it seems the smaller the file, the lesser the black. this is how conversions save space.
as for light grey or blue as you stated in my guide thread, thats extreme and somethings wrong.
i just double checked on the movie 28 weeks later, the beginning of the movie has tons of black colors. the black is not a dark crisp black, but a “filmy black”. (not sure if this is the right descriptive word)