If the composite output of the Amiga is correctly connected to the composite input of your TV card, and the *CORRECT* input is selected in SuperTV (or Elbox TV), you should get a black or grey screen, because even when using P96 modes, AGA is outputting a normal 15Khz signal that the TV card should detect.
If you are getting a blue screen, the TV card is not detecting any signal from the AGA output. Double check your connections, and that the TV software has the correct input selected.