You'll lose too much flavor and aroma if you add it to the primary. Racking your beer on top of the fruit in the secondary is the best bet. Just be sure to use a blow off tube.
As for the amount. It really depends on how much fruit flavor you're looking for. I feel for a generally balanced beer, you can use 4lbs of frozen/fresh blackberries or one 3lb can of blackberry puree. The puree is probably the easiest way to go since it already santitary, and all seeds removed. You use less because it's more concentrated.
If you go the fresh/frozen option, i recommend pasturizing, especially with the fresh fruit. (I've never had an issue just plopping the frozen stuff in the secondary before.) If using fresh, put them in a pot of 150F water for about 20 minutes. Cool and you're good to go.