Add the blueberries. lemon zest and water to a medium (2 quart) heavy-bottomed saucepan set over medium-high heat. Bring to a boil, lower heat and simmer until the blueberries soften and burst, about 5 to 7 minutes. Use a spoon to press berries and break apart berries as they cook.
Strain into a bowl through a fine mesh sieve, pressing pulp with a spoon. Make sure to scrape the puree on the underside of the strainer into the bowl. Allow puree to cool. Discard any unstrained pulp, reserve the puree and rinse the saucepan and sieve.
In a large bowl (or a stand mixer fitted with the paddle attachment), beat the butter and sugar until fully combined, about 2 min. Add the eggs and yolks one at a time, beating each until incorporated. On low, mix in the lemon juice, blueberry puree, salt and cornstarch. The mixture will look curdled, but it will smooth out as it cooks.
Pour the mixture back into the saucepan and cook over low heat until thickened (about 10 minutes), stirring constantly. Don’t walk away or you will have lemony scrambled eggs. The curd will thicken at about 170 degrees F, or just below simmer. Remove from the heat and strain into a bowl through the rinsed sieve.
Add curd to storage jars and allow to cool to room temperature before refrigerating. Curd will get thicker as it cools. If using in tarts right away, allow to come to room temperature before filling shells.