In a saucepan, combine the blueberries and sugar over medium heat. Cook until the blueberries are soft and the sugar has dissolved, about 5 minutes.

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

In a separate bowl, whisk together the eggs and lemon juice until well combined.

Strain the mashed blueberry mixture through a fine-mesh strainer into a clean saucepan, discarding the solids.

Add the egg mixture to the blueberry juice in the saucepan and stir to combine.

Cook over medium-low heat, stirring constantly, until the mixture thickens and coats the back of a spoon, about 10 minutes.

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

Transfer the blueberry curd to a jar or container and let it cool to room temperature.

Once cooled, refrigerate for at least 2 hours before using. The curd will thicken as it cools.