Place the zest and juice of the blood oranges in a small saucepan, along with the sugar, egg yolks and 1 whole egg. Over a medium heat, cook while whisking continuously until the sugar dissolves.
Continue cooking while whisking for 15 minutes as the mixture thickens. You are looking for this to be thick enough to coat the back of a spoon.
Once thickened, remove from the heat and add the butter. Whisk until this has melted and the curd is smooth.
Strain through a sieve to remove the zest and allow the curd to cool before serving.