We have the exact same tv and love it!!!
The black screen as you're changing channels is the tv adjusting to the resolution of the new channel... If you observe in the upper left hand corner, you see the resolution you're currently watching - get a black screen while changing - when it lands on the new channel the resolution change will appear in the same location. The length of time you have black screen will depend on the resolution size differences... I learned this from one of Time-Warner's tech folks.
I had the cable company put in a high definition box and they used an HDMI cable. When I change HD channels, I get a black screen for a few seconds and in the upper left hand corner a box says HDMI input with the screen resolution. Also, there is a slight freeze in the picture on other digital channels when I first change the channel. Can anything be done to correct this? Thanks in advance.