Slice the butter into tablespoon-sized pieces, then set it in the bottom of a mixing bowl with a sieve over it. Set aside.

In a small pot, add the blueberries, orange zest, and orange juice. Cook on medium-low heat for about 20 minutes, stirring as needed, until most of the blueberries have burst and are completely softened, releasing their juices.

Strain this through a fine-mesh sieve into a bowl to remove the blueberry skins and press out all the liquid. 

Rinse and wipe out the pot you used before, then dry it thoroughly. Pour the strained blueberry liquid into the pot and add the sugar.

In a bowl, use a fork to beat the eggs with the salt. Pour into the pot with the sugar and blueberry liquid. Whisk to combine.

Cook over medium-low heat, stirring constantly until thickened, and the temperature reaches 170 F.

Pour through the sieve over the butter. Stir until butter is melted and curd is smooth.

Store in the fridge in an airtight container. Curd should keep for about three weeks. If planning to store long-term, place the curd in a freezer-safe container and seal tightly. Will keep for up to 12 months.