The "zante currants" sold in the grocery stores are NOT actual currants, they're really just raisins made from zante grapes. Here is a link for pure black currant juice: http://www.knudsenjuice.com/products..._black_currant
I've never tried currants/juice in a stout, but I've experimented with the juice in wine. There's a LOT of ascorbic acid in black currants, something like 250-300% the USRDA per serving. I had to use calcium carbonate to try to reduce the acid.
Currants have a great flavour & I can easily see how they could go well in a stout recipe. I've tried the cassis (black currant) lambic & it's quite tasty. I use huckleberries with robust porter in a BBQ sauce recipe, I'll bet huckleberries or blueberries would work well in a stout too.
You might try some taste testing by adding a measured amount of juice to a measured amount of stout, that should give you a basic idea as to the amount to add. Regards, GF.
EDIT: I'd think a sweet stout recipe would be the best to try adding currants/juice to; the non-fermentable sugars will help to balance the acid from the fruit/juice. Regards, GF.