My friend Catherine preserves blueberries in this sweet and tangy sauce. It is so lovely to make these pickled fruits and keep them for Christmas to give as gifts or to serve alongside a selection of cheeses. It's also delicious with duck.
Make a spice bundle by placing the cinnamon sticks, cloves and allspice berries in the centre of a 30cm square piece of muslin cloth. Gather the edges of the muslin together and tie with kitchen string to secure. Place spice bundle into a large saucepan and pour in the vinegar. Bring to a simmer over medium heat; cook for 5 minutes. Stir blueberries into the vinegar; cook until heated through, about 5 minutes. As the blueberries heat, gently shake the pot. Do not stir to avoid breaking the berries. Remove from heat, cover, and let stand at room temperature for 8 to 12 hours.
Pour berries and liquid into a colander or sieve set over a bowl. Remove spice bundle. Transfer berries to hot, sterilised jam jars; reserve the liquid. Return liquid to saucepan and place over high heat. Stir in the granulated and brown sugar; bring to the boil. Boil until thickened, about 4 minutes.
Ladle hot syrup over berries, stopping about 1cm from the top of the jars. Wipe the rims of the jars with kitchen paper to remove any jam residue. Close and seal the jars with sterilised lids.
Place a rack in the bottom of a large pot and fill halfway with water. Bring to the boil over high heat, then carefully lower the jars into the pot. Leave about 5cm space between the jars. Pour in more boiling water if necessary until the water level is at least 2.5cm above the tops of the jars. Bring the water to a rolling boil, cover the pot, and process for 15 minutes.