Next time it happens, try removing your SD card at boot. That way, BootMii won't run. Also, try moving your Wii Remote around in front of the sensor bar at the black screen and see if it rumbles to indicate you are over a button (i.e., to see if the Wii is actually working, but can't display the image. You can try selecting a channel too, and see if you get banner noise to play.
I suspect this might be BootMii forcing the Wii into a slightly different display mode which your TV can't handle.I don't know how that can possibly happen, though...