Take 1 pound of blood oranges (approximately 4-5 tennis ball-sized oranges) and wash them well. Trim away both ends and slice the oranges in half. Using a very sharp knife, trim away the core of the oranges and pluck out any seeds that you find. Set the cores and the seeds aside. Not all blood oranges have seeds, so don't stress if you don't find any. Cut the orange halves into thin slices. Go as thin as you can manage. Finally, cut each sliced half in half again, so that you have a number of thin blood orange quarters. Bundle up all those seeds and pithy cores in a length of cheesecloth and tie it tightly so that nothing can escape. Put chopped oranges in a medium bowl and cover with 3 cups water. Tuck the cheesecloth bundle into the bowl and cover the whole thing with a length of plastic wrap or a plate. Refrigerate it overnight. When you're ready to cook your marmalade, remove the cheesecloth bundle. Combine the soaked fruit and water with 2 1/2 cups granulated sugar. Make sure to fully dissolve the sugar into the fruit before pouring it into the pan. Bring the marmalade to a simmer and cook until it is reduced by more than half, reads 220 degrees F on a thermometer and passes the plate/sauce/wrinkle test. When it is finished cooking, pour marmalade into prepared jars. Wipe rims, apply lids and rings and process in a boiling water bath canner for 10 minutes. When all is done, you should have three half pints of the most vivid red blood orange marmalade!