Puree 2.5 lbs of blackberries at 180F and filter the skins. Add blackberry puree to secondary and ferment for 2 - 3 weeks @ 65F
At bottling time add as much or little of blackberry extract as you like to bottling bucket with finished beer. About 2 oz will give it a perfect aroma and mild sweetness. I would suggest to slowly add 1 - 2 TBSP at a time, stir, taste and repeat until you feel you have what you want.