SandFarmer, I have seen the exact same thing happen to me with my HWS receiver. I have had this happen occasionally, even before the last software update. I have not been able to pin down the exact cause, but sometimes I think it occurs when I press buttons too fast for the HWS to respond to each push.
When the screen goes black, the sound still works, and I can view video on the other tuner by using the swap button. If something is recording on the black screen, the recording works fine. Sometimes, I can get the video back by playing around with the interface (switching to guide, settings, etc, and then going back to the video) but that is hit or miss and I can not reproduce it reliably.
If something is recording while this is happening, I wait until it is done, then reset the receiver. Resetting the receiver always brings back the video on the black screen.