In a small saucepan, combine the blueberries and lemon juice, cover, and place over medium heat. Cook until the blueberries pop and the liquid bubbles, about 5 minutes. Pass the blueberries through a strainer, pressing well with a spoon. Be sure to scrape all the brilliant purple puree on the underside of the strainer into the bowl to use. Pour the blueberry puree back into the saucepan and set aside to cool.
Whisk together the egg yolks and egg until combined. While whisking, pour in sugar and whisk until a light yellow color. Add the egg mixture to the blueberry puree in the sauce pan, and continue whisking until creamy and well incorporated, about one minute. Place over medium heat, whisking constantly, until the custard thickens and can coat the back of a wooden spoon, about 8-10 minutes. Remove the pan from the heat, and stir in the butter, one cube at a time. Strain into a bowl or jars, cover surface with plastic wrap and chill in the refrigerator for at least one hour.
