I’d think there are many pretty drinks to be made with grape juice. Here’s a sample of ideas. http://bartenderapp.com/ingredients/grape-juice/cocktails/list_by_ingredient
I think you need to consider how sophisticated - or not - you want these cocktails to be. A grape juice cocktail isn’t sophisticated to me. Just saying…
Can you muddle blackberries? Might create too many floating pieces. Perhaps process them first to remove seeds and then use the puree in some way?
This drink is too pink b/c of the Chambord, but could be adapted.
1/2 oz Chambord, 2 oz vodka, muddle a few blackberries, fill with ice and lemonade
You could make a blackberry simple syrup and as I mentioned earlier infused vodka or gin. Takes a little extra work but you can do it ahead of time and its not too difficult. Voila! Craft cocktails that you’d have to pay $15 for elsewhere. 
You could go with a wop – usually these are made with fruit punch flavor instead of grape, but grape could work:
- Rum or vodka (or economize with Ambur 190)
- Sprite, 7-Up, Sierra Mist or grape soda
- Grape juice, grape Hawaiian Punch, or grape sports drink
- Sliced fruit and grapes (apples, oranges, watermelon, peaches, strawberries, whatever floats your boat)
Is this a frat party or a wedding? 
Well, D was an honorable mention. She made a great cocktail with soju, sprite, blackberry flavored brandy, ginger syrup and basil leaves. It was delightful but more difficult to figure out the exact proportions than the winner whose drink was 1/3 soju, 1/3, 1/3 (not sure what the other two ingredients were). Since D and I like soda better than alcohol, I think her drink was heavier on the soda than the booze; the guests are likely to prefer more alcohol. Soju is alcohol that is clear and colorless and vodka-like. The wedding venue will allow the hosts to bring in a cocktail made of that (without corkage fee) because the venue doesn’t have it in its bar.
I know soju is Korean. Is it popular in Hawaii, @HImom? I haven’t heard of it much here in the States.
Well, before we went to Korea in 2015 on a tour, I had NEVER heard of Soju, but H and his buddies all knew of it. H had spent many weeks in Korea for work, so he was a lot more familiar with the country than I am. I’m a very featherweight drinker, so I am not a good judge. D was somewhat familiar with soju (tho she is not that much of a drinker) and there are quite a few folks in HI of Korean ancestry (including one of my nephews-in-law).