Well, if the must is so stunned from the .5 teaspoon of k-meta that it wouldn't ferment, you may have an uncarbonated sweet blackberry beer if it never does take off.
I'm certain that .5 teaspoon of k-meta in a 1 gallon batch is a misprint. You use 1/4 teaspoon or so in the must of a 5 gallon batch. It does disipate with time, so you may get it to start up again. I'd try doing it separately before I possibly ruined my beer, though.
If you stir, stir, stir, the blackberry must several times a day, you might get much of the sulfite out and then you can pitch some fresh yeast on it.