Fill a large bowl half way with ice. Pour cold water over the ice so that the bowl is no more than ⅔ high. Place a clean bowl on top of the water bath. Place a strainer over the empty bowl. Set aside
Pour half the sugar (you can eyeball half) into a 2 or 3 quart saucepan. Add the lemon zest and press it into the sugar with your fingertips. Add the milk and cream and whisk over low to medium heat, stirring occasionally to ensure the sugar dissolves. Heat to a simmer.
While the liquid heats up, vigorously whisk the egg yolks with the remaining sugar. You want to whisk it until the mixture turns pale yellow, becomes thick, and forms a ribbon when dripped on itself. (You can use an electric mixer with the whisk attachment if you are feeling lazy. It will take approximately 5 minutes of constant whisking by hand.)
Once the liquid mixture comes to a simmer, turn the heat off and let it sit for a minute or two. With the bowl containing the egg/sugar mixture secure on the counter (if no one is around to hold it, place it on a rubber mat or wet a dishtowel and wrap it around the base of the bowl), slowly pour the heated liquid mixture into the eggs, whisking constantly. Once fully incorporated, pour the contents of the bowl back into the saucepan. Over a medium-low heat, stir the mixture constantly with a wooden spoon until it reaches 185° F or a line holds when you drag your finger across the back of the wooden spoon. (It should only take a minute or two to reach the correct temperature.)
Immediately remove the saucepan from the heat and pour the cooked mixture through the strainer into the empty bowl sitting in the ice water bath. Lift strainer and discard the zest. Stir in the vanilla and lemon juice. Continue stirring every few minutes as it sits in the ice bath until the mixture has completely cooled, about 10 minutes.
Pour into a container. Cover with plastic wrap directly over (touching) the mixture. Cover and place in refrigerator for 4-24 hours.
Combine all the blueberry swirl ingedients into a small saucepan. Cook over medium heat, stirring constantly, about 5 minutes. The mixture should be deep puple and slightly thicken. Remove from heat and transfer to a heatproof container. Allow to cool to room temperature. Mix in vodka, if using. Cover and refrigerate until ready to use.
Follow your ice cream manufacturer’s directions to churn the ice cream. Transfer the ice cream to a freezer container, layering it with the blueberry swirl. Cover with a piece of parchment paper or plastic wrap directly on the surface. Cover container and freeze.