Sorry for late answer, been on vacation.
I checked and I can't find any "HDMI Black Level" settings on my TV.
Does it matter that I have a plasma TV? I can only find information about those settings on LCD displays.
I also tried to connect my display to the reciever with component cables today (and the xbox still hooked up with component to the reciever), but it doesn't make any difference.
The image is still dark, and if I turn the brightness and contrast levels up, it get's to bright and gray.