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1
Add berries and the lemon juice to a blender or food processor fitted with the chopping blade; blend or process until fairly smooth.
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2
Transfer to a fine-mesh sieve set over a big bowl; gently press the puree against the mash with the back of a wooden spoon to extract as much of the juice and pulp as possible, leaving the skins behind.
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3
Discard the mass in the sieve and set the puree aside.
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4
Heat the milk and cream in a saucepan over medium heat until tiny bubbles fizz around the pan's inner rim; adjust heat to maintain this temperature without letting the mixture boil.
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5
Beat the egg yolks and sugar in a medium bowl with an electric mixer at medium speed until pale yellow, thick and grainy, about 2 minutes.
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6
Whisk about one quarter of the hot milk mixture into the egg yolk mixture until smooth, then whisk this combined mixture into the remaining hot milk mixture in the pan.
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7
Immediately decrease heat to low; cook slowly, stirring constantly, until the mixture thickens to the consistency of melted ice cream and can coat the back of a wooden spoon, about 4 minutes.
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8
Strain through a clean fine-mesh sieve into a medium bowl to remove any bits of scrambled egg; stir in the blueberry puree, vanilla, and salt.
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9
Refrigerate until cold, about 4hours or overnight.
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10
Freeze the custard in your ice cream machine according to manufacturer's directions.
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11
Serve at once or scoop into a freezer container; store in the freezer for up to 1 month; soften at room temperature for up to 10 minutes before serving.
