Thoroughly wash 12 blood oranges under running water. Cut six oranges into quarters, then slice into thin strips, retaining the juice. Place the sliced oranges in a large saucepan, cover with water, and bring to a boil over medium heat. Once boiling, remove from heat, cover, and let them sit overnight for at least 8 hours to soften.

The next day, strain the soaked orange rinds and pulp using a fine mesh strainer, placing them into a Dutch oven. Juice the remaining six blood oranges, and combine their juice with enough of the soaking liquid to total about 4 cups.

Add the juice mixture to the Dutch oven along with 8 cups of granulated sugar. Squeeze in fresh lemon juice from 2-3 lemons, ensuring you capture that zesty acidity. Stir well until the sugar is fully dissolved.

Over medium-high heat, bring the mixture to a rolling boil, stirring continuously until the sugar is completely dissolved. Once boiling, reduce the heat to medium and simmer uncovered for about 20 minutes.

To check for doneness, use a candy thermometer to ensure the mixture reaches 220°F (104°C) or do the cold plate test. Spoon a small amount onto a plate that's been chilled in the freezer for a few minutes; it should gel upon cooling.

Carefully ladle the hot Blood Orange Marmalade into sterilized canning jars, leaving about ¼ inch of headspace at the top. Wipe the rims of each jar to remove any residue, then apply the lids securely.

After processing, remove the jars from the canner and let them cool undisturbed on a clean, dry surface for 24 hours. Check the seals by pressing down on the center of each lid.