If there is a blackout, can I still use electric energy?

If you have an energy exchange system, the answer is absolutely NO.
If there was electricity when a blackout happens, even if for maintenance reasons, all the workers would be electrocuted. There would be the same danger if you wanted to maintain your own system and forget to unplug the PV system when you switch electricity off with the master switch.

The system is not able to see if electricity has been unplugged for maintenance reasons (in your house or in the network) or if there actually is a blackout. That’s why there are laws, which require an immediate and automatic unplug of the PV system when the inverter sees discrepancies or absence of voltage.