Over medium heat, bring the blood orange juice to a simmer in a small saucepan. Simmer until the juice is reduced by half, about 15 minutes. (Reducing the juice will intensify its flavor.) Allow to cool to room temperature.

In a heatproof bowl, beat the zest, yolks, and sugar with a fork until slightly lighter in color. Add the reduced blood orange juice, lemon juice and salt and stir to combine.

Bring 1 inch of water to a simmer in a saucepan over medium heat. Fit the heatproof bowl with the egg mixture into the saucepan, being sure the bowl doesn’t touch the water directly (this will form a double boiler.)

Cook the egg mixture over the simmering water, stirring nearly constantly, until it has thickened, about 10–20 minutes. (The temperature of the mixture should read about 170F when it is finished, and a spoon dragged through should leave a trail.)

Stir in the butter 1 tbsp at a time, melting each completely before adding more. Remove the curd from the heat and strain through a fine-mesh sieve (working through with a spoon) into a storage jar.