My unqualified opinion...
Rather than a blank serving as a warning (also serving as a complete disregard for the K.I.S.S. principle); if you really want to warn an intruder in hopes of avoiding a deadly encounter, why not simply call out and advise that you will defend your home and family with deadly force if they do not cease, desist and leave immediately? Probably not a bad idea to mention the cops are on their way (which they should be).
At least this way you've in no way compromised your ability to defend your home and family (i.e., by employing useless blanks), and at the same time you can rest easy knowing you've tried your hardest to scare the intruder away so that you don't have *possibly* to end his/her life.
Being discovered will most likely scare a majority of intruders away -- my opinion, not fact. It is also not something I'm willing to gamble on, so I find it best to have a firearm ready to defend if this does not work. Having a firearm only capable of making noise, does not fit the "defend" criteria.
Nothing wrong with trying to avoid killing people; it's a refreshing point of view, actually... but there is something wrong with accepting a disadvantage or otherwise compromising your ability to defend, in order to avoid killing someone. All is fair in love and war. If someone brings war to your doorstep, you must be prepared to battle. If someone brings you love, well, then load your blanks and celebrate.