sweaten with apple juice or the other juice

I have a system I like for cider or cyser. At bottling I add one can of apple juice concentrate per gallon, bottle with one bottle being plastic. When plastic bottle sufficiently hard I pasteurize. I don't like chemicals so I use new bottled juice so no campden.
What if I sweetened with the other juice I used, like the one I want to do next, blackberry?
What would be the effect of fermenting with the blackberry vs sweetening with it? What if I used half the blackberry to ferment, froze the other half for sweetening?

 

As far as I recall, blackberries are not really high in sugar. But to actually answer your question, if you add the berry juice right before pasteurizing, the berry flavor will be solid, and up front.
I just re read your post, Yes, if you add blackberries in the beginning, and then again at pasteurizing, I am sure the flavor will be fantastic, but maybe a little too tart.

 

I'm doing a similar thing with pure Vermont maple syrup, added in primary and when it's done I'll back sweeten with it and keg it . It should be interesting