In a small heavy-bottomed saucepan, combine the fresh blueberries, 1/4 cup granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the blueberries burst and the mixture thickens slightly, about 8-10 minutes.

Remove from heat and mash the blueberries with a fork or potato masher to create a chunky sauce. Let cool completely to room temperature, then transfer to a small bowl, cover, and chill in the refrigerator for at least 1 hour, or until cold.

In a medium heavy-bottomed saucepan, combine the heavy cream, whole milk, and salt. Heat over medium heat until the mixture just begins to simmer and small bubbles form around the edges. Do not boil. Remove from heat.

In a large mixing bowl, whisk together the 3/4 cup granulated sugar and egg yolks until light in color and slightly thickened.

Slowly temper the hot cream mixture into the egg yolk mixture: gradually whisk about a cup of the hot cream into the egg yolks to raise their temperature without scrambling them. Then, slowly pour the egg yolk mixture back into the saucepan with the remaining hot cream, whisking constantly.

Return the saucepan to medium-low heat. Cook, stirring constantly with a rubber spatula, until the custard thickens enough to coat the back of a spoon (it should reach about 175-180°F / 79-82°C). Do not boil, or the custard may curdle.

Remove from heat and immediately strain the custard through a fine-mesh sieve into a clean large bowl to remove any cooked egg bits and ensure a smooth base. Stir in the vanilla extract.

Cover the bowl with plastic wrap, pressing it directly onto the surface of the custard to prevent a skin from forming. Chill the custard in the refrigerator for at least 4 hours, or preferably overnight, until thoroughly cold.

Once the ice cream base is completely chilled, churn it in your ice cream maker according to the manufacturer's instructions until it reaches the consistency of soft-serve ice cream.

Spoon about one-third of the churned ice cream into a freezer-safe container. Drizzle generously with one-third of the chilled blueberry swirl. Repeat layers with remaining ice cream and blueberry swirl, gently swirling with a knife or spoon for a marbled effect.

Cover the container tightly and freeze for at least 4-6 hours, or until firm, before serving.