Both TiVo HD boxes have this problem. Certain HD channels show black when you turn to them. The guide works fine. Also, it is the same channels on both boxes that don't work. I.E. TruTV HD appears black on both boxes.
About 20% of HD channels appear Black. No Tivo warning message comes up, it is just black.
I have restarted both boxes and they still have the same problem.
I use an M-Card in both boxes. San Diego - Cox Cable.