Combine the blueberries, 1/4 cup of sugar and lemon zest together in a saucepan over medium heat. Stir occasionally, being mindful that the pot does not boil over or burn. Keep the berries on medium heat until they release their juices and the sugar is completely dissolved. Remove from heat and strain, pressing the skins to release all the juices. Add the lemon juice and stir to combine. Once the mixture has cooled to room temperature, place in a sealed bowl to chill for least two hours.
In a saucepan over medium heat, whisk together the milk, cream and salt. Cook until bubbles form around the edges of the pan. Whisk together the egg yolks and sugar until thick and pale yellow, for about five minutes. Drizzle in the olive oil and beat until combined, scraping down the sides of the bowl occasionally.
Slowly add 2 cups of the hot milk mixture, 1/4 cup at a time, beating until combined. Pour the yolk mixture back into the saucepan, whisking constantly. Place the pan over medium-low heat and cook, continuously whisking, until the mixture coats the back of a spoon or until it reaches 175°F.
Strain the custard through a fine-mesh sieve into a bowl. Place the bowl in a larger one partially filled with ice water and cool the custard to room temperature, stirring occasionally. Cover with plastic wrap and refrigerate until cold, about 2 hours.
Add the chilled blueberry puree to the custard and stir to combine. Place the mixture in to the ice cream maker and process according to the manufacturer's instructions. Transfer the gelato to a chilled container, cover and freeze until firm, at least 3 hours, before serving.