I too had a problem with HDCP and think I figured out what the problem is.
Hopefully these steps will help people having Blu-Ray "Black Screen" Playback Issues or DVD Playback Error code 0x91d70001 and DO NOT have a capture card plugged into the console (Also reported to cause these errors, due to HDCP protection)
After lots of troubleshooting, uninstalling the Blu-Ray App, Power Cycling the console etc. I basically switched the HDMI cable to another HDMI port on my TV and BOOM! I haz movies.
I then set about trying to get movies to work over HDMI Port 1 again. I factory reset my TV and plugged the Xbox back into HDMI Port 1 on my TV and it now works fine.
It seems like there's some sort of HDCP bug with the Console, and it blocked that HDMI port? I'm not sure, but either way, this fixed my problem.
Hope it helps!