I think it will depend on the TV and the method of connecting the HDR (Scart or HDMI). My stab in the dark would be Panasonic and HDMI but please tell us and perhaps there's someone with the same setup who can help you.
I assume that the scenario is: you are watching TV with a scheduled recording and the Hummy in standby and the blank screen appears when the Hummy comes out of sleep mode to record the programme (But still not fully awake). If your scenario is not as I have guessed at, perhaps you could describe it a bit more concisely than you have done above, so that we can fully understand what's going on.
Does your TV auto switch to the Hummy when you switch on the Hummy?
PS I have Sony TV with HDMI connection and it certainly does not do anything like that.