Maybe you could use something like this, just don't bother to overlay any text. Not sure if it will re-generate the full video signal if no background is available or it expects a signal to genlock on to.
Maybe you could use an LM1881 to supply sync info to an arduino and if it stops supplying a sync output (no video?) then get the arduino to clamp and stamp sync signals on this feed to stop the TV blue screen (What model is it as you can turn off the blue screen on some models)