Thoroughly wash the outside of each blood orange. Top and tail the oranges (remove the ends) then slice in half. Trim away the core of each orange and remove any seeds. Reserve core and seeds. Thinly slice each half widthwise then slice in half lengthwise.

Place chopped oranges in a large bowl and cover with water. Bundle up all cores and seeds in a small square of cheesecloth. Be sure to tie the ends tightly so nothing comes out. Submerge the bundle into the bowl of slices and water. Cover the bowl and refrigerate overnight.

Remove the cheesecloth bundle and discard. Pour blood orange slices and water into a large pot. Add sugar, stirring to combine. Bring mixture to a boil over medium-high heat then reduce to low and let simmer until reduce by more than half, stirring occasionally. Test the consistency of the marmalade by placing 1 tablespoon of the mixture on small plate. Place the plate in the refrigerator for 10 minutes. If you find the marmalade is a bit too runny after 10 minutes, return the saucepan to the heat and simmer away for another 5 – 10 minutes. Repeat this process until desired consistency is achieved.

Transfer hot marmalade to sterilized jars (pour boiling water over the jars and lids) and seal. Once the jars have cooled, store in the refrigerator.