In a colander, rinse watermelon rinds well under cold water and allow to drain.
In a tea filter or a square of cheesecloth, wrap the crushed cardamom pods, whole cloves, bay leaves, and whole peppercorns together. Seal or tie off the sachet d’épices.
In a large, non-reactive pot, combine the apple cider vinegar, reserved watermelon juice, the Redpath® Dark Brown Sugar, Redpath® Granulated Sugar, sliced ginger, cinnamon stick, thyme, strips of lime zest, and the sachet d’épices.
Bring the mixture to a boil, stirring to dissolve the sugars. Lower heat, cover with a lid, and simmer for 10 minutes to infuse flavours.
Carefully add the watermelon rinds to the hot liquid.
Bring the mixture back to a boil. Reduce heat to low, cover, and gently simmer rinds until translucent; about 30 minutes. Stir occasionally.
Remove pot from heat. With the help of tongs or a slotted spoon, remove ginger slices, cinnamon stick, thyme, lime zest, and the sachet d’épices. Discard.
Cool watermelon rinds to at least room temperature. Add in blackberries; stir to distribute.
Transfer cooled rinds and berries to an airtight container or jars, ensuring pickles are submerged in the pickle solution. For best flavour, allow pickles to stand in the refrigerator for at least 2 days; consume within a month. The pickled rinds and berries should be kept refrigerated.