I've made a blackberry porter and a raspberry porter. Both were extract recipes.
- For the blackberry porter, I racked 5 gallons of beer (post-fermentation) over 3 lbs of (thawed) frozen blackberries. Let it sit two weeks, and bottled. The result was a really fine porter, with almost no detectable blackberry flavor, and no aroma.
- Second attempt was a raspberry porter. I added a 32-oz. can of pureed raspberry directly to the fermenter when fermentation was about 80% complete. Let it sit a week, then racked to secondary, let it clear, and then bottled. Result is a very distinct raspberry flavor *and* aroma.
Honestly, it's a touch too strong for me, but all my friends like it. And I got what I was after: a raspberry porter. I think the puree really lets the flavor come out.
Next time, I'd love to try a blackberry puree.
YMMV