Set aside 2 cups of the blackberries to remain whole.
2 cups Frozen Blackberries 1/3 cup Sugar Juice and Zest of 1/2 Lemon 2 tablespoons Cornstarch
Combine the remaining blackberries, 1/2 of the sugar, and lemon juice and zest, and cornstarch in a food processor. Pulse until pureed. Taste and adjust sweetness if necessary. Put mixture in a small sauce pot and simmer on medium-low heat for about 12-15 minutes, until it has cooked down and become thick.
2 cups Heavy Cream 1/2 teaspoon Vanilla Extract
In a medium bowl, beat the cream and vanilla extract until peaks form and cream is whipped. Divide the whipped cream into 2 equal portions.
In a large trifle dish add 1 cup of the berry mixture to the bottom. Take the rest of the berry mixture and fold it into 1 of the whipped creams (leaving the other plain). Begin layering the trifle dish with the plain whipped cream followed by some fresh blackberries and the berry cream. Repeat until you've used all ingredients. Garnish with fresh blackberries.