If you use 100% blackberries, they have enough tannin on their own to make a pretty darn good wine. I've never used elderberries, so I have no idea what they taste like. If you like the taste of them, though, you could use them. A small amount might really be good in the wine. Of course, you could also use the grape concentrate!
Sorry I don't seem to be much help here! But wine is very forgiving, and truly you can't screw it up. If the elderberries have a taste you like, go ahead and use them, especially if it's a small amount. A larger amount would have lots of tannin and probably be too much, unless you were going to age it for a couple of years. I've made blackberry wine with 100% blackberries, and it's awesome. But if I didn't have enough blackberries, I would not hesitate to use some grape juice to make it up. (Remember, though, if you use grape juice to use the 100% grape juice and make sure there are no preservatives in there.) Whatever you do, you'll get a homemade wine that will taste good, maybe great. Just take really good notes on what you've done, so that you can use this as a learning experience!