My Solar PV system correctly shuts off power during a blackout - I want to safely override that
If your inverter shuts down completely when there is no line power, it is probably not capable of putting out AC without AC present.  Some inverters assume the power line is essentially a 0 &Omega; source.  They look at the voltage and decide what current to dump onto it.  These types of inverters don't actually synthesize the 60 Hz themselves.  They count on that being a input.

If you want to be self-powered during a blackout, you have to get the right inverter for that, which probably costs more.  Then the inverter also has to deal with not being able to hold up the line to the proper voltage, and what to do about it.  It gets complicated, which is why many inveters require the voltage waveform to already be there.