@Cherry Puffling, A more pertinent question is "why do we add tannin to fruit wines?"
Among the reasons are increased structure, increased mouthfeel, better taste, and longer shelf life. The first 3 points run together -- the tannin gives the wine a "backbone" that improves flavor. The last point is that tannin is an anti-oxidant the protects the wine and gives it a longer lifespan.
Some fruit wines taste like Kool-Aid to me, having sugar, flavor, and alcohol, but lacking in that "oomph" that grape wines have. Look at a lot of fruit wine recipes -- they call for adding tea, a source of tannin.
Elderberries have tannin levels pretty close to red grapes. Blackberries and blueberries are supposed to be close to red grape levels.
To answer your question -- I don't expect that adding 250 ml grape concentrate to a 6 liter batch of blackberry is going to be a problem. In your other thread I suggested doing 1 batch with and 1 without -- that will provide you with an answer that may apply to all future batches.
Obviously the more love and precision you put into it the less likely your going to be suprised with the end result or have something nasty happen.
Exactly. Some of the guys that taught me pulled lugs of grapes off the train (I was in northern New York state, grapes came from California by train), crushed them, threw them in the barrel, and let nature take its course. Some years this wine was the best -- unfortunately, just as often the wine was mediocre and sometimes poor. OTOH, the guys that put more structure into winemaking consistently produced good wines, often exceptional ones.
A professional winemaker commented that, "Wine makes itself. We merely guide it along." Or words to that effect. I've found that to be very true.
