In a saucepan, combine the blueberries and sugar. Cook over medium heat until the blueberries release their juices, about 5 minutes.

Remove from heat and mash the blueberries with a fork or potato masher.

In a bowl, whisk together the eggs and lemon juice. Gradually add the blueberry mixture, whisking continuously.

Return the mixture to the saucepan and cook over low heat, stirring constantly until thickened, about 10 minutes.

Remove from heat and stir in the butter until melted and smooth.

Strain the curd through a fine-mesh strainer into a clean jar to remove any solids.

Let cool to room temperature, then refrigerate until set.