Measure the milk into heavy bottomed sauce pan.
Bruise and crush 1 cup of lemon basil leaves and add to pot.
Heat the milk and lemon basil on medium low to medium heat and bring to a low simmer, stirring constantly to prevent scorching.
Remove the pot from the heat and allow to slightly cool.
While the milk is cooling combine the egg yolks, sugar, corn syrup and salt in a large bowl. Whisk together until the yolks become a light creamy yellow.
Strain the cooled milk and the discard lemon basil leaves.
Temper the yolk mixture and warm milk by slowly whisking about ⅓ of a cup of the warm milk into the egg yolks.
Slowly pour the remaining milk into the bowl of the tempered egg yolks and whisk until milk and egg mixture is combined.
Pour the custard base back into the pot.
Bring the custard mixture to a low boil on medium heat, stirring constantly.
Cook until thickened, the custard is done when a trail can be seen on the back of a spoon when you run your finger through the custard.
Strain the custard through a fine mesh strainer into a clean bowl, cover and chill in the refrigerator until cold, about 1 to 2 hours.
While the custard is cooling pulse the blackberries, lemon juice and the reserved ½ cup of lemon basil leaves in a blender until the berry pulp is smooth.
Pour the blackberry pulp through a fine mesh strainer placed over a bowl.
Set aside and let the juice from the berries drain from the pulp while custard cools.
Remove the gelato custard from the refrigerator and stir in the strained blackberry juice, add in about a teaspoon of pulp if you would like some seeds in your gelato or leave the seeds out if you prefer.
Process in an ice cream maker as directed by manufacturer.
Serve immediately or scoop gelato into an airtight container and freeze.