Wash the blood oranges and dry them first.
Grate the rind to get as much orange grind as possible, but avoid grating the white pith under the skin. You might not need all the zest, but you can also use it for other recipes.
Juice all the blood oranges and pour the juice through a fine mesh sieve to get rid of any pith, pips or large pulp.
Add about 1 inch or 2 cm of water to a medium size saucepan and bring to gentle simmer over a low to medium heat on your hob.
In a suitable bowl (glass or heat proof plastic) mix the sugar, lemon juice (if using) blood orange zest and blood orange juice with a large pinch of salt.
Place the bowl over the saucepan and let the sugar and butter dissolve first. Stir gently to make sure all ingredients are incorporated and the sugar has dissolved properly (check on the back of a clean spoon for any sugar crystals).
In the meanwhile, separate the eggs into egg whites (keep for other recipes) and egg yolks.
Once the sugar and butter has dissolved, turn the heat right down (or even turn it off completely) add the eggs to the main curd mixture.
Carry on stirring the blood orange curd mixture, ensuring that the heat is very low and that the water doesn't boil (only simmers).
Stir gently for another 20 minutes or until the mixture thickens and becomes smooth and creamy.
To achieve a smooth blood orange curd, pour and press the hot mixture through a fine sieve (and discard any large parts).
Pour carefully into prepared jam jars or other suitable containers.
Keep in a refrigerator and use within 1-2 weeks.