I do not agree with topping this off with another wine (although a Shiraz would probably blend nicely with the blackberries). With as much headspace as this has, you would very much change the character of the wine, and you made blackberry wine for a reason.
As suggested, I would top off -- either with marbles (although, admittedly, that would take a lot of marbles!)....or purge the oxygen with CO2 or (preferably) nitrogen. That will also work, and will be much easier racking. When I've used the marble trick from time to time, you lose a fair amount of wine in between the marbles, because it can be tricky to get to.