This is actually a problem, during a blackout the grid tie inverter will still source power, but it probably won't be enough to power the loads (houses) around you and will send current back out to the line and cause problems:
First and foremost is the technical reason. The electronics that
control a solar system constantly adjust voltage and current to keep
the panels operating efficiently as the sunlight changes. To do this,
the system produces quantities of power that aren’t dependent on how
much your house is actually using in a given moment. In a
grid-connected system, any excess power is put back onto the grid for
others to use, and your utility credits you on your bill for that
Solar power output varies directly with sunlight levels. So, even if
you disregard the need for efficiency, connecting this variable
resource directly to your home’s electrical system would cause your
lights to blink, damage your refrigerator, and wreak havoc on your
computers and television.
The second reason that solar shuts down during a blackout is safety.
During a power outage, the power utility sends out repair crews to
find and fix the points of failure. Linemen and women will be
jeopardized if there is a local power generator (like a solar array)
leaking power onto the grid lines. Therefore, utility rules mandate
that in the event of a power outage, solar arrays must automatically
shut down. Solar systems have detectors that sense whether power is
coming across the grid, and whenever grid power is down, they
automatically shut down too, to protect utility workers.
For this reason most (but not all) grid tie inverters don't source power when the grid goes down. There are some that can detect grid faults and disconect the building power from the grid during a blackout.