Sounds like an all-too-drinkable brew, like the "Snakebite and Black" we used to drink as teenagers (cider, beer and blackcurrant syrup - yes, dear oh dear, to be blamed for many a youthful misdeed...)
Let us know what it's like.
Is that store-bought cider? Seems a shame to use fresh apple juice in an experiment like that, not sure there would be much of a difference. We have frozen black cherries round here, I think I would be tempted to chuck a load of those in the secondary one time.