In a medium saucepan, combine 2 cups of fresh blackberries, 1 cup of granulated sugar, and 1 cup of water. Cook over medium-high heat for about 10 minutes, mashing the blackberries with a fork until the mixture thickens and becomes syrupy. Once bubbling, strain the mixture through a fine sieve to remove the seeds and pulp, then stir in 1 teaspoon of vanilla extract. Allow the syrup to cool completely.

While the syrup is cooling, juice about 6 fresh lemons until you have approximately 1 cup of lemon juice. In a 2-quart pitcher, combine the freshly squeezed lemon juice with 4 cups of cold water. Stir well to mix.

Once the blackberry syrup has cooled, add it to the pitcher of lemonade. Start with about 1 cup of blackberry syrup, then taste and adjust as needed to achieve your desired sweetness. Stir thoroughly.

Refrigerate your blackberry lemonade for at least 30 minutes before serving. Serve over ice and garnish each glass with fresh blackberries or a lemon slice.