blackberry stout is sour

So, I took my Black Sand Stout and kegged half of it (it's waiting. . . we're waiting. . . ) and put the other half on 3lb of frozen blackberries. We wanted to do boysenberries but they're out of season and we could NOT find any. . . anywhere. . . sigh.

so they've been on the blackberries for about a week and I popped a thief in there (properly sterilized and all that) and stole a few ounces. . .

it's sour. smoky, coffee flavors, but sour . . .

We were hoping for more sweet.

Is this something I have to change the recipe for? More Crystal to get more unfermentable sugars? that's what I'm surmising, but wanted to get some opinions. . .

And yes, I know, relax - I'm not panicking or anything, just checking. . .

I'd say go with your gut add more unfermentable sugar next time. My experience with berry/fruit beers has been similar.

When berries have the sugars taken away (fermented out), they are very sour.

You could try adding a pound of lactose to your stout, or more or less to taste, and that should sweeten it up.

Thanks Yooper- and kudos to your avatar.

DD - I will look into that for my next iteration of this stout