Smart Juice Organic
When flavoring your tea (see my guide to double fermentation) there really is no regimented way to go about it. Just try things that you think will make the flavor unlike any other drink. I had good luck with this flavor of not from concentrate juice from Smart Juice Organic. I have only compared their Mulberry to a flavor they have called Antioxidant Force which is a blend of about 6 different organic not from concentrate juices.
Just be sure to pick something organic, in the freshest possible format you can get it. Its probably fine if you dont, but the idea is that the more processed the juice you add, the more the chance that a wild chemical made by some mad scientist will hurt your double ferment. Maybe we should experiment with that in the future!